add GL_INCLUDE_FILE capability

WFObj.cc

@@ -8,7 +8,6 @@
 #include <ctype.h>      /* isspace() */
 #include <string.h>     /* strlen() */
 #include <stdlib.h>     /* atoi */
-#include <GL/gl.h>
 
 #include <vector>
 #include <string>

WFObj.h

@@ -2,7 +2,12 @@
 #ifndef WFOBJ_H
 #define WFOBJ_H
 
+#ifdef GL_INCLUDE_FILE
+#include GL_INCLUDE_FILE
+#else
 #include <GL/gl.h>
+#endif
+
 #include <vector>
 #include <string>
 #include <map>