Cross-compile to Windows (64-bit) is failing given that statically built OpenSSL can't be found

[TSC21]

I am trying to cross-compile a C++ project to Windows using dockcross (more specifically, https://github.com/dockcross/dockcross/tree/master/windows-static-x64) in Ubuntu 20.04. This container image in specific uses MXE together with the x86_64-w64-mingw32 compiler.

OpenSSL is also built (statically) through this container using the following configuration and build command: (note this is being called from CMake, and so the usage of CMake variables)

# Config
perl <SOURCE_DIR>/Configure mingw64 no-shared no-tests --prefix=${CMAKE_INSTALL_PREFIX}
# build
RC=/usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-windres
AR=/usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-ar
RANLIB=/usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-ranlib
CROSS_COMPILE=x86_64-w64-mingw32.static-
make -e RC=${RC} AR=${AR} RANLIB=${RANLIB} CROSS_COMPILE=${CROSS_COMPILE}

The command above runs properly and libcrypto.a and libssl.a are both installed in ${CMAKE_INSTALL_PREFIX}/lib, besides the headers on the ${CMAKE_INSTALL_PREFIX}/include folder. Here's the install tree structure:

.
├── bin
│   ├── ...
│   └── openssl.exe
├── include
│   ├── ...
│   └── openssl
│      ├── ...
│      └── ssl.h
├── lib
│   ├── ...
│   ├── libcrypto.a
│   └── libssl.a
...

But, for some reason, on the C++ project, and when calling find_package(OpenSSL REQUIRED), it fails to find OPENSSL_CRYPTO_LIBRARY as it can be seen above. Note that OPENSSL_INCLUDE_DIR had to also be passed as a CMAKE_ARG to the project because FindOpenSSL() is was also not able to find openssl/ssl.h, although it is installed in ${CMAKE_INSTALL_PREFIX}/include. OpenSSL is marked as a dependency of libdatachannel while building through CMake, so I don't necessarily understand why this fails. One thing to note is that MXE sets MSYS to 1, and also since we are using mingw32 to cross-compile,WIN32 and MINGW are also set to 1. If I try to manually set MINGW and WIN32 to 0, then find_package(OpenSSL) succeeds and the build process continues, until the linker fails because the architecture is of course wrong. Setting the OPENSSL_ROOT_DIR also doesn't help.

This is the error seen:

-- Compiler flags (CMAKE_C_FLAGS):  -std=c99 -pedantic -Wall -Wextra -Wfloat-equal -Wshadow -Wpointer-arith -Wunreachable-code -Winit-self -Wno-unused-function -Wno-unused-parameter -Wno-unreachable-code -Wstrict-prototypes -Werror
-- link library: ws2_32
-- Configuring done
CMake Error at /usr/src/mxe/usr/x86_64-pc-linux-gnu/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the
  system variable OPENSSL_ROOT_DIR (missing: OPENSSL_CRYPTO_LIBRARY) (found
  version "1.1.1o")
Call Stack (most recent call first):
  /usr/src/mxe/usr/x86_64-pc-linux-gnu/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  /usr/src/mxe/usr/x86_64-pc-linux-gnu/share/cmake-3.20/Modules/FindOpenSSL.cmake:571 (find_package_handle_standard_args)
  deps/libsrtp/CMakeLists.txt:75 (find_package)

Is this potentially a problem with the OpenSSL build or even with the default FindOpenSSL.cmake module? Or is this something that can be addressed on the C++ project CMakeLists.txt? Any help with this would be appreciated.

Thanks in advance!

[TSC21]

For whatever it can help with, here's the /usr/src/mxe/usr/x86_64-w64-mingw32.static/share/cmake/mxe-conf.cmake MXE configuration:

# This file is part of MXE. See LICENSE.md for licensing information.

# https://cmake.org/cmake/help/latest

# Can't set `cmake_minimum_required` or `cmake_policy` in toolchain
# since toolchain is read before CMakeLists.txt
# See `target-cmake.in` for CMAKE_POLICY_DEFAULT_CMPNNNN

# Check if we are using mxe supplied version
#   - toolchain is included multiple times so set a guard in
#     environment to suppress duplicate messages
if(NOT ${CMAKE_COMMAND} STREQUAL /usr/src/mxe/usr/x86_64-pc-linux-gnu/bin/cmake AND NOT DEFINED ENV{_MXE_CMAKE_TOOLCHAIN_INCLUDED})
    message(WARNING "
** Warning: direct use of toolchain file is deprecated
** Please use prefixed wrapper script instead:
     x86_64-w64-mingw32.static-cmake [options] <path-to-source>
       - uses mxe supplied cmake version 3.20.1
       - loads toolchain
       - loads common run results
       - sets various policy defaults
    ")
    set(ENV{_MXE_CMAKE_TOOLCHAIN_INCLUDED} TRUE)
endif()

# Use CACHE variables to allow user setting with `-D`
# Use CACHE FORCE in rare cases of misconfigured CMakeLists.txt
#   - e.g include(FindPkgConfig)
#     https://github.com/mxe/mxe/issues/1023
#   - projects may still set these in which case FORCE doesn't have
#     any advantage, just causes inconvenience
#     https://github.com/mxe/mxe/pull/1621#discussion_r106937505
# Use normal variables expected to be set by toolchain/system
#   - projects should test for these values and not try to override

## General configuration
set(CMAKE_SYSTEM_NAME Windows)
set(CMAKE_SYSTEM_PROCESSOR x86_64 CACHE STRING "System Processor")
set(MSYS 1)
set(CMAKE_EXPORT_NO_PACKAGE_REGISTRY ON)
# Workaround for https://www.cmake.org/Bug/view.php?id=14075
set(CMAKE_CROSS_COMPILING ON)


## Library config
set(BUILD_SHARED_LIBS OFF CACHE BOOL "BUILD_SHARED_LIBS")
set(BUILD_STATIC_LIBS ON CACHE BOOL "BUILD_STATIC_LIBS")
set(BUILD_SHARED OFF CACHE BOOL "BUILD_SHARED")
set(BUILD_STATIC ON CACHE BOOL "BUILD_STATIC")
set(LIBTYPE STATIC)


## Paths etc.
# These MODEs shouldn't be changed by users, we only want headers/libs
# from cross-build and "never" want binaries. We do, however, want
# `*-config` scripts but there's no way to instruct cmake to do that.
#
# The best solution may be to whitelist utilities
#   https://github.com/mxe/mxe/issues/1667
# and symlink them to an additional root path, changing PROGRAM to ONLY

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

# Allow user to specify list of locations to search
set(CMAKE_FIND_ROOT_PATH /usr/src/mxe/usr/x86_64-w64-mingw32.static CACHE PATH "List of root paths to search on the filesystem")
set(CMAKE_PREFIX_PATH /usr/src/mxe/usr/x86_64-w64-mingw32.static CACHE PATH "List of directories specifying installation prefixes to be searched")
set(CMAKE_INSTALL_PREFIX /usr/src/mxe/usr/x86_64-w64-mingw32.static CACHE PATH "Installation Prefix")
# For custom mxe FindPackage scripts
set(CMAKE_MODULE_PATH "/usr/src/mxe/usr/share/cmake/modules" ${CMAKE_MODULE_PATH})

# projects (mis)use `-isystem` to silence warnings from 3rd-party
# source (among other things). gcc6 introduces changes to search
# order which breaks this usage.
#   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
#   https://gitlab.kitware.com/cmake/cmake/issues/16291
#   https://gitlab.kitware.com/cmake/cmake/issues/16919
set(CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES /usr/src/mxe/usr/x86_64-w64-mingw32.static/include)
set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES /usr/src/mxe/usr/x86_64-w64-mingw32.static/include)


## Programs
set(CMAKE_C_COMPILER /usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-gcc)
set(CMAKE_CXX_COMPILER /usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-g++)
set(CMAKE_Fortran_COMPILER /usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-gfortran)
set(CMAKE_RC_COMPILER /usr/src/mxe/usr/bin/x86_64-w64-mingw32.static-windres)
# CMAKE_RC_COMPILE_OBJECT is defined in:
#     <cmake root>/share/cmake-X.Y/Modules/Platform/Windows-windres.cmake
set(CPACK_NSIS_EXECUTABLE x86_64-w64-mingw32.static-makensis)

## Individual package configuration
file(GLOB mxe_cmake_files
    "/usr/src/mxe/usr/x86_64-w64-mingw32.static/share/cmake/mxe-conf.d/*.cmake"
)
foreach(mxe_cmake_file ${mxe_cmake_files})
    include(${mxe_cmake_file})
endforeach()
set(CMAKE_CROSSCOMPILING_EMULATOR "/usr/bin/wine")