Revert "Cmake: add compiler option in a more idiomatic way"

This reverts commit 183995.

It broke the bots:
http://bb.pgr.jp/builders/cmake-clang-x86_64-linux/builds/9730/steps/build_clang/logs/stdio

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183997 91177308-0d34-0410-b5e6-96231b3b80d8