Fix GCC build in C++14 w/o c++14 constexpr

llvm-svn: 337974
diff --git a/libcxx/src/experimental/filesystem/filesystem_common.h b/libcxx/src/experimental/filesystem/filesystem_common.h
index 5c18930..a74cae4 100644
--- a/libcxx/src/experimental/filesystem/filesystem_common.h
+++ b/libcxx/src/experimental/filesystem/filesystem_common.h
@@ -229,7 +229,7 @@
           .count();
 
 private:
-#if _LIBCPP_STD_VER > 11
+#if _LIBCPP_STD_VER > 11 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
   static constexpr fs_duration get_min_nsecs() {
     return duration_cast<fs_duration>(
         fs_nanoseconds(min_nsec_timespec) -