build: remove unnecessary modification of CMAKE_REQUIRED_DEFINITIONS
build: remove unnecessary modification of CMAKE_REQUIRED_DEFINITIONS

This is unnecessary now that the flag handling has been fixed.  The flags will
be added properly in the main CMakeLists.txt after the config-ix inclusion which
performs the required check.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@203639 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx