[CMake] Allow passing extra CMake arguments to custom libc++

[phosek]

Summary

This can be used to customize the libc++ build.

[vitalybuka]

Can you please add some details about why someone wants to customize LIBXX here?
Or even better attach a patch which uses this.

[phosek]

In D41103#955540, @vitalybuka wrote:

Can you please add some details about why someone wants to customize LIBXX here?
Or even better attach a patch which uses this.

I plan on using it from D37631 where I currently modify the CMAKE_ARGS directly but that's not a very clean solution.

[phosek]

ping?

cmake/Modules/AddCompilerRT.cmake

 # add_custom_libcxx(<name> <prefix>
 #                   DEPS <list of build deps>
 #                   CFLAGS <list of compile flags>)
 macro(add_custom_libcxx name prefix)
   if(NOT COMPILER_RT_LIBCXX_PATH)
     message(FATAL_ERROR "libcxx not found!")
   endif()
 
-  cmake_parse_arguments(LIBCXX "" "" "DEPS;CFLAGS" ${ARGN})
+  cmake_parse_arguments(LIBCXX "" "" "DEPS;CFLAGS;CMAKE_ARGS" ${ARGN})
   foreach(flag ${LIBCXX_CFLAGS})
     set(flagstr "${flagstr} ${flag}")
   endforeach()
   set(LIBCXX_CFLAGS ${flagstr})
 
   if(NOT COMPILER_RT_STANDALONE_BUILD)
     list(APPEND LIBCXX_DEPS clang)
   endif()
 
   ExternalProject_Add(${name}
     PREFIX ${prefix}
     SOURCE_DIR ${COMPILER_RT_LIBCXX_PATH}
     CMAKE_ARGS -DCMAKE_MAKE_PROGRAM:STRING=${CMAKE_MAKE_PROGRAM}
                -DCMAKE_C_COMPILER=${COMPILER_RT_TEST_COMPILER}
                -DCMAKE_CXX_COMPILER=${COMPILER_RT_TEST_CXX_COMPILER}
                -DCMAKE_C_FLAGS=${LIBCXX_CFLAGS}
                -DCMAKE_CXX_FLAGS=${LIBCXX_CFLAGS}
                -DCMAKE_BUILD_TYPE=Release
                -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
                -DLLVM_PATH=${LLVM_MAIN_SRC_DIR}
                -DLIBCXX_STANDALONE_BUILD=On
+               ${LIBCXX_CMAKE_ARGS}
     LOG_BUILD 1
     LOG_CONFIGURE 1
     LOG_INSTALL 1
     )
   set_target_properties(${name} PROPERTIES EXCLUDE_FROM_ALL TRUE)
 
   ExternalProject_Add_Step(${name} force-reconfigure
     DEPENDERS configure