Add backend/gl/glx.c to build

Still a long way to go for the glx backend.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/gl_common.h

@@ -66,10 +66,13 @@ gl_create_program_from_str(const char *vert_shader_str, const char *frag_shader_
 
 bool gl_load_prog_main(session_t *ps, const char *vshader_str, const char *fshader_str,
                        gl_win_shader_t *pprogram);
+void gl_free_prog_main(session_t *ps, gl_win_shader_t *prog);
 
 unsigned char *gl_take_screenshot(session_t *ps, int *out_length);
 void gl_resize(int width, int height);
 
+GLuint glGetUniformLocationChecked(GLuint p, const char *name);
+
 /**
  * Get a textual representation of an OpenGL error.
  */