meson: Only use vendored glext.h if there is no system glext.h

The vendored glext.h is needed on Windows, but on Linux gl.h will
already include a much newer version of glext.h, which causes a version
mismatch.

Wrangling OpenGL across platforms is a mess and maybe we should just
use glew for this, but for now this works.

Author: arch1t3cht

meson.build

@@ -361,6 +361,8 @@ if not dep_gl.found()
 endif
 if host_machine.system() == 'darwin'
     conf.set('HAVE_OPENGL_GL_H', 1)
+else
+    conf.set('HAVE_GL_GLEXT_H', cc.has_header('GL/glext.h'))
 endif
 
 if not dep_gl.found()

src/gl_wrap.cpp

@@ -28,8 +28,12 @@
 #include <OpenGL/glext.h>
 #else
 #include <GL/gl.h>
+#ifdef HAVE_GL_GLEXT_H
+#include <GL/glext.h>
+#else
 #include "gl/glext.h"
 #endif
+#endif
 
 static const float deg2rad = 3.1415926536f / 180.f;
 static const float pi = 3.1415926535897932384626433832795f;