[compiler-rt] Fix a bug in the cmakelists file when CMAKE_CXX_FLAGS are empty

[stella.stamenova]

Summary

Right now, the regex expression will fail if the flags were not set. Instead, we should follow the pattern of other llvm projects and quote the expression, so that it can work even when the flags are not set.

[phosek]

LGTM, thanks!

compiler-rt/CMakeLists.txt

 set(COMPILER_RT_COMMON_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 set(COMPILER_RT_COMMON_LINK_FLAGS ${SANITIZER_COMMON_LINK_FLAGS})
 
 # We don't use the C++ standard library, so avoid including it by mistake.
 append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_NOSTDLIBXX_FLAG -nostdlib++ SANITIZER_COMMON_LINK_FLAGS)
 
 # Remove -stdlib= which is unused when passing -nostdinc++...
-string(REGEX MATCHALL "-stdlib=[a-zA-Z+]*" stdlib_flag ${CMAKE_CXX_FLAGS})
-string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+string(REGEX MATCHALL "-stdlib=[a-zA-Z+]*" stdlib_flag "${CMAKE_CXX_FLAGS}")
+string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 
 # ...we need it to build some runtimes and tests so readd it where appropriate.
 list(APPEND COMPILER_RT_COMMON_CFLAGS ${stdlib_flag})
 list(APPEND COMPILER_RT_COMMON_LINK_FLAGS ${stdlib_flag})
 
 macro(append_libcxx_libs var)
   if (${var}_INTREE)
     if (SANITIZER_USE_STATIC_LLVM_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))