Implement the infrastructure for feature-test macros. Very few actual feature test macros, though. Reviewed as: https://reviews.llvm.org/D51955

llvm-svn: 342073
diff --git a/libcxx/include/cmath b/libcxx/include/cmath
index ffb1c46..f5f62ad 100644
--- a/libcxx/include/cmath
+++ b/libcxx/include/cmath
@@ -303,6 +303,7 @@
 
 #include <__config>
 #include <math.h>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header