CMAKE: help CheckAtomic find check_library_exists

[martell]

Summary

lldb fails to build standalone on x86

  -- Performing Test HAVE_CXX_ATOMICS64_WITHOUT_LIB - Failed
  CMake Error at CheckAtomic.cmake:66 (check_library_exists):
    Unknown CMake command "check_library_exists".
  Call Stack (most recent call first):
    LLDBStandalone.cmake:90 (include)
CMakeLists.txt:3 (include)

[martell]

add context so reviewers can see where it is used

[martell]

ping

[rnk]

lgtm

llvm/trunk/cmake/modules/CheckAtomic.cmake

 # atomic builtins are required for threading support.
 
 INCLUDE(CheckCXXSourceCompiles)
+INCLUDE(CheckLibraryExists)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
 
 function(check_working_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "-std=c++11")
   CHECK_CXX_SOURCE_COMPILES("