[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

llvm-svn: 338531

libcxx/include/__config

@@ -510,7 +510,7 @@ namespace std {
 #define _LIBCPP_HAS_IS_BASE_OF
 #endif
 
-#if !__EXCEPTIONS
+#if !__EXCEPTIONS && !defined(_LIBCPP_NO_EXCEPTIONS)
 #define _LIBCPP_NO_EXCEPTIONS
 #endif