[libc++] Fix GCC 7.2.0 macro redefinition warning
[libc++] Fix GCC 7.2.0 macro redefinition warning

The warning happens when LIBCXX_ENABLE_EXCEPTIONS cmake option is not set,
and it fires every time __config is included, 33 in total.

Patch by Jason Lovett
Reviewed as https://reviews.llvm.org/D49997

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@338531 91177308-0d34-0410-b5e6-96231b3b80d8

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