honor Python2_EXECUTABLE and Python3_EXECUTABLE when they are passed to cmake

[robertu94]

Summary

CMake's find_package(Python3) and find_package(Python2) packages have a PYTHON_EXECUTABLE, Python2_EXECUTABLE, and Python3_EXECUTABLE cmake variables which control which version of python is built against. As far as I can tell, the rest of LLVM honors these variables. This can cause the build process to fail when if the automatically selected version of Python can't run due to modifications of LD_LIBRARY_PATH when using spack. The corresponding Spack issue is https://github.com/spack/spack/issues/19908. The corresponding LLVM issue is 48180

I believe an appropriate fix is to add the variables to the list of PASSTHROUGH_VARIABLES in cmake/Modules/AddCompilerRT.cmake, and this fixed compilation errors for me.

This bug affects distributions like Gentoo and package managers like Spack which allow for combinatorial versioning.

[Meinersbur]

There might be other cmake configuration parameters used for introspection, such as ZLIB_INCLUDE_DIR (if using zlib), <PackageName>_ROOT etc.

In this case, it seems clear to me that the path to python should be passed to the inner build configuration. LGTM.

If you need someone to commit this, please say so.

[robertu94]

I don't have commit privileges.

[Meinersbur]

Thank you for you the fix!

compiler-rt/cmake/Modules/AddCompilerRT.cmake

set(PASSTHROUGH_VARIABLES
     CMAKE_LINKER
     CMAKE_AR
     CMAKE_RANLIB
     CMAKE_NM
     CMAKE_OBJCOPY
     CMAKE_OBJDUMP
     CMAKE_STRIP
     CMAKE_SYSROOT
+    PYTHON_EXECUTABLE
+    Python3_EXECUTABLE
+    Python2_EXECUTABLE
     CMAKE_SYSTEM_NAME)
   foreach(variable ${PASSTHROUGH_VARIABLES})
     get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)
     if(${is_value_set})
       get_property(value CACHE ${variable} PROPERTY VALUE)
       list(APPEND CMAKE_PASSTHROUGH_VARIABLES -D${variable}=${value})
     endif()
   endforeach()