C99 at least. C89 still fails due to the use of block comments.
NOTE: Having libc++ on the include path when compiling C is not
recommended or ever really supported. However it happens often
enough that this change is warrented.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305539 91177308-0d34-0410-b5e6-96231b3b80d8