build: use standard CMAKE_INSTALL_INCLUDEDIR for header install folder

[OlivierLDff]

This gives user control over which folder header should be installed to instead of using hardcoded value include/. Variable CMAKE_INSTALL_INCLUDEDIR is provided after a call to include(GNUInstallDirs)
More info can be found about usage in docs: https://cmake.org/cmake/help/latest/command/install.html

This PR basically replace all $<INSTALL_INTERFACE:include> with $<INSTALL_INTERFACE:include>.

---

It was already correctly used here:

date/CMakeLists.txt

Line 173 in 22ceabf

PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/date )

here:

date/CMakeLists.txt

Line 178 in 22ceabf

DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/date )

and here:

date/CMakeLists.txt

Line 184 in 22ceabf

PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/date

---

This fix a bug if user provide -DCMAKE_INSTALL_INCLUDEDIR=customfolder, right now generated CMake/dateTargets.cmake output a file with:

set_target_properties(date::date PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "ONLY_C_LOCALE=0"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_SOURCES "${_IMPORT_PREFIX}/include/date/date.h"
)

With this PR, generated file will be:

set_target_properties(date::date PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "ONLY_C_LOCALE=0"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/customfolder"
  INTERFACE_SOURCES "${_IMPORT_PREFIX}/customfolder/date/date.h"
)