cmake: set SDL2_LIBRARY to an absolute path

Previously the build ignored the prefix for SDL2 discovered via
pkg-config and linking with only -lSDL2 could result in trying to link
to a system SDL2 instead of some custom SDL2 build.

In my case I'm building vogl for 32 bit and 64 bit on a multilib
system and although pkg-config is setup to find a 32bit build of
SDL, the vogl build was trying to link with my system's 64 bit
library.