[KMSDRM] Enable full OpenGL detection (MESA/libglvnd) on both the make and cmake buildsystems.

cmake/sdlchecks.cmake

@@ -777,6 +777,22 @@ macro(CheckVivante)
   endif(VIDEO_VIVANTE)
 endmacro(CheckVivante)
 
+# Requires:
+# - libglvnd
+macro(CheckOpenGLKMSDRM)
+  if(VIDEO_OPENGL AND HAVE_VIDEO_KMSDRM)
+    check_c_source_compiles("
+        #include <GL/gl.h>
+        int main(int argc, char** argv) {}" HAVE_VIDEO_OPENGL)
+
+    if(HAVE_VIDEO_OPENGL)
+      set(HAVE_VIDEO_OPENGL TRUE)
+      set(SDL_VIDEO_OPENGL 1)
+      set(SDL_VIDEO_RENDER_OGL 1)
+    endif()
+  endif()
+endmacro()
+
 # Requires:
 # - nada
 macro(CheckOpenGLX11)