[CMake] Don't override required compiler flags in the runtimes build

[phosek]

Summary

Ensure that HandleLLVMOptions adds all necessary required flags,
including -Wno-error when building with LLVM_ENABLE_WERROR enabled.

[phosek]

This is a better alternative to D57142. After this change lands, runtimes build should be working with LLVM_ENABLE_WERROR.

[smeenai]

LGTM

llvm/runtimes/CMakeLists.txt

105 ↗

(On Diff #186382)

Does DetermineGCCCompatible not do the right thing already?

llvm/trunk/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   # This variable makes sure that e.g. llvm-lit is found.
   set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR})
   set(LLVM_CMAKE_PATH ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
   if(APPLE)
     set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
   endif()
 
-  set(SAFE_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(SAFE_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
-
   include(CheckLibraryExists)
   include(CheckCCompilerFlag)
 
-  check_library_exists(c fopen "" LLVM_HAS_C_LIB)
-  check_c_compiler_flag(-nodefaultlibs LLVM_HAS_NODEFAULTLIBS_FLAG)
-  if(LLVM_HAS_NODEFAULTLIBS_FLAG)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
-    if(LLVM_HAS_C_LIB)
-      list(APPEND CMAKE_REQUIRED_LIBRARIES c)
-    endif()
-  endif()
+  # We don't have libc++ (yet).
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 
   # Avoid checking whether the compiler is working.
   set(LLVM_COMPILER_CHECKED ON)
+  # Mark the compiler as compatible with GCC.
   # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions
   # resulting in unjustified successes by check_cxx_compiler_flag.
   set(LLVM_ENABLE_WARNINGS ON)
 
   # Handle common options used by all runtimes.
   include(AddLLVM)
   include(HandleLLVMOptions)
   include(FindPythonInterp)
 
-  set(CMAKE_REQUIRED_FLAGS ${SAFE_CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_LIBRARIES ${SAFE_CMAKE_REQUIRED_LIBRARIES})
+  # Remove the -nostdlib++ option we've added earlier.
+  string(REPLACE "-nostdlib++" "" CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
 
   # This can be used to detect whether we're in the runtimes build.
   set(RUNTIMES_BUILD ON)
 
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
 
     # TODO: Clean this up as part of an interface standardization