Quick regression fix for SDL2 detection.

I only had to add "PATH_SUFFIXES SDL2".
.
include is superfluous
SDL2-2.0 is a distro specific convenience symlink. Use the correct one
as before.
.
I probably did this as an artificial way of disabling pkgconfig when cross
compiling and before disabling pkgconfig for all cross compiling but it's
wrong. I thought I had fixed this already.