[cmake] Fix native tooling when cross-compiling on Linux

[smeenai]

Summary

At least as of CMake 3.20.3, the CMake platform file for Linux doesn't
define the file type prefix and suffix variables, relying on them being
implicitly empty when they're unset. If we're cross-compiling targeting
Windows on a Linux machine, the values of these prefixes and suffixes
populated by the Windows platform file will still be set after including
the Linux platform file, so we'll incorrectly assume the ".exe" suffix
for the host machine. Explicitly unset the variables before including
the platform file, to prevent any previous values from leaking. Thanks
@beanz for suggesting the fix.

[beanz]

Yep, that's exactly what I was suggesting. Seems to work as expected for me locally.

[smeenai]

In D108473#2957693, @beanz wrote:

Yep, that's exactly what I was suggesting. Seems to work as expected for me locally.

Yep, works for me too. Thanks for the quick review!

[dfukalov]

Thanks!

llvm/cmake/config-ix.cmake

 
 if(PY_PYGMENTS_FOUND AND PY_PYGMENTS_LEXERS_C_CPP_FOUND AND PY_YAML_FOUND)
   set (LLVM_HAVE_OPT_VIEWER_MODULES 1)
 else()
   set (LLVM_HAVE_OPT_VIEWER_MODULES 0)
 endif()
 
 function(llvm_get_host_prefixes_and_suffixes)
+  # Not all platform files will set these variables (relying on them being
+  # implicitly empty if they're unset), so unset the variables before including
+  # the platform file, to prevent any values from the target system leaking.
+  unset(CMAKE_STATIC_LIBRARY_PREFIX)
+  unset(CMAKE_STATIC_LIBRARY_SUFFIX)
+  unset(CMAKE_SHARED_LIBRARY_PREFIX)
+  unset(CMAKE_SHARED_LIBRARY_SUFFIX)
+  unset(CMAKE_IMPORT_LIBRARY_PREFIX)
+  unset(CMAKE_IMPORT_LIBRARY_SUFFIX)
+  unset(CMAKE_EXECUTABLE_SUFFIX)
+  unset(CMAKE_LINK_LIBRARY_SUFFIX)
   include(Platform/${CMAKE_HOST_SYSTEM_NAME} OPTIONAL RESULT_VARIABLE _includedFile)
   if (_includedFile)
     set(LLVM_HOST_STATIC_LIBRARY_PREFIX ${CMAKE_STATIC_LIBRARY_PREFIX} PARENT_SCOPE)
     set(LLVM_HOST_STATIC_LIBRARY_SUFFIX ${CMAKE_STATIC_LIBRARY_SUFFIX} PARENT_SCOPE)
     set(LLVM_HOST_SHARED_LIBRARY_PREFIX ${CMAKE_SHARED_LIBRARY_PREFIX} PARENT_SCOPE)
     set(LLVM_HOST_SHARED_LIBRARY_SUFFIX ${CMAKE_SHARED_LIBRARY_SUFFIX} PARENT_SCOPE)
     set(LLVM_HOST_IMPORT_LIBRARY_PREFIX ${CMAKE_IMPORT_LIBRARY_PREFIX} PARENT_SCOPE)
     set(LLVM_HOST_IMPORT_LIBRARY_SUFFIX ${CMAKE_IMPORT_LIBRARY_SUFFIX} PARENT_SCOPE)
     set(LLVM_HOST_EXECUTABLE_SUFFIX ${CMAKE_EXECUTABLE_SUFFIX} PARENT_SCOPE)
     set(LLVM_HOST_LINK_LIBRARY_SUFFIX ${CMAKE_LINK_LIBRARY_SUFFIX} PARENT_SCOPE)
   endif()
 endfunction()
 
 llvm_get_host_prefixes_and_suffixes()