constexpr: Use GLIBCXX instead of GNUC (#1288)

Summary:
libcxx does not have strcmp or strlen as constexpr.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Pull Request resolved: https://github.com/facebook/folly/pull/1288

Reviewed By: Orvid

Differential Revision: D19226813

Pulled By: yfeldblum

fbshipit-source-id: 4b5de4f923f4d7871c059af54eb7d375f344f48a

folly/portability/Constexpr.h

@@ -63,7 +63,7 @@ constexpr size_t constexpr_strlen(const char* s) {
 #if FOLLY_HAS_FEATURE(cxx_constexpr_string_builtins)
   // clang provides a constexpr builtin
   return __builtin_strlen(s);
-#elif defined(__GNUC__) && !defined(__clang__)
+#elif defined(__GLIBCXX__) && !defined(__clang__)
   // strlen() happens to already be constexpr under gcc
   return std::strlen(s);
 #else
@@ -81,7 +81,7 @@ constexpr int constexpr_strcmp(const char* s1, const char* s2) {
 #if FOLLY_HAS_FEATURE(cxx_constexpr_string_builtins)
   // clang provides a constexpr builtin
   return __builtin_strcmp(s1, s2);
-#elif defined(__GNUC__) && !defined(__clang__)
+#elif defined(__GLIBCXX__) && !defined(__clang__)
   // strcmp() happens to already be constexpr under gcc
   return std::strcmp(s1, s2);
 #else