build: use pkg-config to find imlib2
build: use pkg-config to find imlib2

imlib2 1.7.5+ drops imlib2-config, hence pkg-config
must now be used to find it.

Bug: https://bugs.gentoo.org/828962
Closes: https://github.com/neagix/idesk/issues/7

neagix/idesk,neagix/idesk,neagix/idesk