Added check for the presence of Mesa's header files.

diff --git a/include/acconfig.h b/include/acconfig.h
index 19e6bba..402ff71 100644
--- a/include/acconfig.h
+++ b/include/acconfig.h
@@ -76,4 +76,4 @@
 #undef HAVE_IPX_LINUX
 
 /* Define if Mesa is present on the system or not */
-#undef HAVE_MESAGL
+#undef HAVE_LIBMESAGL