Only issue compile warning on gcc and msvc

I'm not sure on the proper syntax for clang and mingw

include/assimp/pbrmaterial.h

@@ -49,7 +49,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #ifdef __GNUC__
 #   pragma GCC system_header
 #   warning pbrmaterial.h is deprecated. Please update to PBR materials in materials.h and glTF-specific items in GltfMaterial.h
-#else
+#else if defined(_MSC_VER)
 #   pragma message("pbrmaterial.h is deprecated. Please update to PBR materials in materials.h and glTF-specific items in GltfMaterial.h")
 #endif