Fix GCC 7.2.0 redefinition warning when LIBCXX_ENABLE_EXCEPTIONS cmake option is not set.
llvm/projects/libcxx/include/__config:514:0: warning: "_LIBCPP_NO_EXCEPTIONS" redefined
#define _LIBCPP_NO_EXCEPTIONS<command-line>:0:0: note: this is the location of the previous definition
This warning fires every time __config is included, 33 in total.