fixed compilation for linux

Author: wesgar3

ext/gpupixel/core/gpupixel_macros.h

@@ -67,6 +67,7 @@
 #endif
 #include <gl/GL.h>
 #elif defined(GPUPIXEL_LINUX)
+#include <GL/gl.h>
 #endif
 
 #define NS_GPUPIXEL_BEGIN namespace gpupixel {
@@ -107,4 +108,4 @@
   }
 #else
   #define CHECK_GL(glFunc) glFunc;
-#endif
+#endif

ext/gpupixel/core/opengl3_loader.h

@@ -106,6 +106,10 @@ extern "C" {
 #define GLAPI extern
 #endif
 
+#ifdef GPUPIXEL_LINUX
+#undef GL_VERSION_2_0
+#endif
+
 #include <cassert>
     /* glcorearb.h is for use with OpenGL core profile implementations.
     ** It should should be placed in the same directory as gl.h and