s/HOMEBREW_PREFIX/prefix/ in generated pkg-config .pc files

A number of formulae generate pkg-config .pc files at install time. Some of them use `#{HOMEBREW_PREFIX}`, which results in anything using those .pc files getting the entire Homebrew prefix added to LDFLAGS/CFLAGS/etc. instead of direct paths to the appropriate places in the Cellar.

This commit series fixes the issue in {mad libspotify libid3tag nss} which I think are all afflicted formulae.