Merge #1249: cmake: Add SECP256K1_LATE_CFLAGS configure option

42f8c51 cmake: Add `SECP256K1_LATE_CFLAGS` configure option (Hennadii Stepanov)

Pull request description:

  This PR enables users to override compiler flags that have been set by the CMake-based build system, such as warning flags.

  The Autotools-based build system has the same feature out-of-the-box.

  See more details [here](#1235 (comment)).

  Here are some examples of the new option usage:
  ```
  cmake -S . -B build -DSECP256K1_LATE_CFLAGS="-Wno-extra -Wlong-long"
  ```

  ```
  cmake -S . -B build -DSECP256K1_BUILD_EXAMPLES=ON -DSECP256K1_LATE_CFLAGS=-O1
  cmake --build build
  ...
  In function ‘secp256k1_ecmult_strauss_wnaf’,
      inlined from ‘secp256k1_ecmult’ at /home/hebasto/git/secp256k1/src/ecmult_impl.h:353:5:
  /home/hebasto/git/secp256k1/src/ecmult_impl.h:291:5: warning: ‘aux’ may be used uninitialized [-Wmaybe-uninitialized]
    291 |     secp256k1_ge_table_set_globalz(ECMULT_TABLE_SIZE(WINDOW_A) * no, state->pre_a, state->aux);
        |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from /home/hebasto/git/secp256k1/src/secp256k1.c:29:
  /home/hebasto/git/secp256k1/src/ecmult_impl.h: In function ‘secp256k1_ecmult’:
  /home/hebasto/git/secp256k1/src/group_impl.h:174:13: note: by argument 3 of type ‘const secp256k1_fe *’ to ‘secp256k1_ge_table_set_globalz’ declared here
    174 | static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const secp256k1_fe *zr) {
        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from /home/hebasto/git/secp256k1/src/secp256k1.c:30:
  /home/hebasto/git/secp256k1/src/ecmult_impl.h:345:18: note: ‘aux’ declared here
    345 |     secp256k1_fe aux[ECMULT_TABLE_SIZE(WINDOW_A)];
        |                  ^~~
  ...
  ```

  Please note that in the last case providing `env CFLAGS=-O1` or `-DCMAKE_C_FLAGS=-O1` won't work.

ACKs for top commit:
  real-or-random:
    ACK 42f8c51

Tree-SHA512: 2b152e420a4a8ffd5f67857de03ae5ba9f2223e535ac01a867c1025e0619180d8255fdd1e5fb8279b290f0a1c96bcc874043ef968fcd99b1ff4e13041a91b1e1

CMakeLists.txt

@@ -265,9 +265,14 @@ if(SECP256K1_BUILD_BENCHMARK OR SECP256K1_BUILD_TESTS OR SECP256K1_BUILD_EXHAUST
   enable_testing()
 endif()
 
+set(SECP256K1_LATE_CFLAGS "" CACHE STRING "Compiler flags that are added to the command line after all other flags added by the build system.")
+include(AllTargetsCompileOptions)
+
 add_subdirectory(src)
+all_targets_compile_options(src "${SECP256K1_LATE_CFLAGS}")
 if(SECP256K1_BUILD_EXAMPLES)
   add_subdirectory(examples)
+  all_targets_compile_options(examples "${SECP256K1_LATE_CFLAGS}")
 endif()
 
 message("\n")
@@ -341,6 +346,9 @@ else()
   message(" - LDFLAGS for executables ............ ${CMAKE_EXE_LINKER_FLAGS_DEBUG}")
   message(" - LDFLAGS for shared libraries ....... ${CMAKE_SHARED_LINKER_FLAGS_DEBUG}")
 endif()
+if(SECP256K1_LATE_CFLAGS)
+  message("SECP256K1_LATE_CFLAGS ................. ${SECP256K1_LATE_CFLAGS}")
+endif()
 message("\n")
 if(SECP256K1_EXPERIMENTAL)
   message(

cmake/AllTargetsCompileOptions.cmake

@@ -0,0 +1,12 @@
+# Add compile options to all targets added in the subdirectory.
+function(all_targets_compile_options dir options)
+  get_directory_property(targets DIRECTORY ${dir} BUILDSYSTEM_TARGETS)
+  separate_arguments(options)
+  set(compiled_target_types STATIC_LIBRARY SHARED_LIBRARY OBJECT_LIBRARY EXECUTABLE)
+  foreach(target ${targets})
+    get_target_property(type ${target} TYPE)
+    if(type IN_LIST compiled_target_types)
+      target_compile_options(${target} PRIVATE ${options})
+    endif()
+  endforeach()
+endfunction()