nvizlib: define GL_CLAMP_TO_EDGE if not defined (requires opengl >= 1.2)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@49252 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/nviz/draw.c

@@ -16,6 +16,10 @@
 
 #include <grass/nviz.h>
 
+#ifndef GL_CLAMP_TO_EDGE
+#define GL_CLAMP_TO_EDGE 0x812F
+#endif 
+
 static int sort_surfs_max(int *, int *, int *, int);
 
 /*!