Makefile + configure changes (use pkg-config)

I made this work better on OS X, using pkg-config where available. However, there are still some issues with CPATH and LIBRARY_PATH, and the following must be done also (depending on where your SDL is installed -- you can get this from pkg-config, however this shouldn't be necessary...):

`bash-3.2$ export LIBRARY_PATH="/opt/local/lib"
bash-3.2$ export CPATH="/opt/local/include"
bash-3.2$ make`