Don't link to libGL at compile time

Get all OpenGL functions using SDL_GL_GetProcAddress(). This makes it
easier to cross-arch compile on Linux and add support for OpenGL ES
in the future.

Users still have to supply their own libSDL2 for cross-arch compiling
on Linux. But now the user does not have to re-install libgl1-mesa-dev
package for i386 or amd64 on Debian when switching between compiling
ioquake3 for x86 and x86_64.

code/renderergl2/tr_init.c

@@ -1036,7 +1036,7 @@ void GfxInfo_f( void )
 		GLint numExtensions;
 		int i;
 
-		glGetIntegerv(GL_NUM_EXTENSIONS, &numExtensions);
+		qglGetIntegerv(GL_NUM_EXTENSIONS, &numExtensions);
 		for (i = 0; i < numExtensions; i++)
 		{
 			ri.Printf(PRINT_ALL, "%s ", qglGetStringi(GL_EXTENSIONS, i));