Use pkg-config instead of sdl2-config
Resolves build issue on Arch Linux (and Arch-derivatives)
(Also forms part of a fix for Slackware-current builds)
Accepted By: echotangoecho (Arch)
Confirmed as functional by: Imarok (Ubuntu 16.04, gcc5.4), wes-fole-dog (Parabola)
Refs: #5157
Fixes: #5152
Differential Revision: https://code.wildfiregames.com/D1582