[glibmm] ustring: Silence warning if MSC_VER is undefined/0

[Daniel Boles <dboles src gnome org>]

commit 72a19fe0ad28a5f3643d3ed26a58179330f49d0c
Author: Daniel Boles <dboles src gnome org>
Date:   Tue Jun 2 16:06:00 2020 +0100

    ustring: Silence warning if MSC_VER is undefined/0

 glib/glibmm/ustring.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/glib/glibmm/ustring.h b/glib/glibmm/ustring.h
index f0be919f..dad8d549 100644
--- a/glib/glibmm/ustring.h
+++ b/glib/glibmm/ustring.h
@@ -30,7 +30,7 @@
 #include <type_traits>
 
 /* work around linker error on Visual Studio if we don't have GLIBMM_HAVE_ALLOWS_STATIC_INLINE_NPOS */
-#if (_MSC_VER >= 1600) && !defined (GLIBMM_HAVE_ALLOWS_STATIC_INLINE_NPOS)
+#if defined(MSC_VER) && MSC_VER >= 1600 && !defined(GLIBMM_HAVE_ALLOWS_STATIC_INLINE_NPOS)
 const std::basic_string<char>::size_type std::basic_string<char>::npos = 
(std::basic_string<char>::size_type) -1;
 #endif