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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337974 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/experimental/filesystem/filesystem_common.h b/src/experimental/filesystem/filesystem_common.h
index 5c18930..a74cae4 100644
--- a/src/experimental/filesystem/filesystem_common.h
+++ b/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) -