[CMake][runtimes] Use -nodefaultlibs for the runtimes build

[phosek]

Summary

We may not have a working C++ standard library at this point so we shouldn't rely on it when running CMake checks.

[beanz]

LGTM!

llvm/trunk/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   endfunction()
 
   cmake_minimum_required(VERSION 3.4.3)
 
   # Add the root project's CMake modules, and the LLVM build's modules to the
   # CMake module path.
   list(INSERT CMAKE_MODULE_PATH 0
     "${CMAKE_CURRENT_SOURCE_DIR}/../cmake"
-    "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/Modules"
+    "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/modules"
     "${LLVM_BINARY_DIR}/lib/cmake/llvm"
   )
 
   # Some of the runtimes will conditionally use the compiler-rt sanitizers
   # to make this work smoothly we ensure that compiler-rt is added first in
   # the list of sub-projects. This allows other sub-projects to have checks
   # like `if(TARGET asan)` to enable building with asan.
   foreach(entry ${runtimes})
if(${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})
 
   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()
+
   # Handle common options used by all runtimes.
   include(AddLLVM)
   include(HandleLLVMOptions)
 
+  set(CMAKE_REQUIRED_FLAGS ${SAFE_CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_LIBRARIES ${SAFE_CMAKE_REQUIRED_LIBRARIES})
+
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
 
     # TODO: Clean this up as part of an interface standardization
     string(REPLACE "-" "_" canon_name ${projName})
     string(TOUPPER ${canon_name} canon_name)
     # The subdirectories need to treat this as standalone builds
     set(${canon_name}_STANDALONE_BUILD On)