I added -I/usr/local/include to $CFLAGS and -L/usr/local/lib to
$LDFLAGS for the Darwin build, so that it would find the jpeg and
tiff libs on my system.

and -L/usr/X11/lib to $LDFLAGS when building on OS X. This will ensure that
it picks up the built-in PNG library.

This reverts commit 13dbbd0f.

to an absolute path, so that the rake-compiler task can see those
files.