[CMake] Find SDL

[Christer Solskogen]

Hi!

I have a cross compiler, installed into /opt/cross, which is compiled by 
me. This cross compiler (gcc) is sysroot aware, which means that every 
header and library is installed into /opt/cross/<target>.

In order to get cmake to find SDL (both SDL1 and SDL2) I need to specify 
SDLDIR. The project I'm using (called hatari, a Atari ST(e) emulator) is 
also using other libraries like readline and png, which cmake have no 
problem finding.

Is this a bug in cmake? Right now the cmake version I'm using is 
2.8.12.2, but this problem have been there since I can remember.

-- 
chs