build: fix add_definition abuse in CMake

add_definitions is meant for adding C preprocessor definitions.  Modern cmake
suggests use of the CMAKE_CXX_FLAGS for the purposes of pushing flags to the
compilation commands.  Simply switch to the modern form given that we are
already requiring a new enough cmake.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203637 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed