CI: utility to compare semantically pc files between 2 dirs.

While we are providing both autotools and meson generation, don't compare generated pc files lexically, but semantically: autotools generates files with ${variable} while meson, not providing the same things, can define the same way, but differently (expending some variables instead or providing other variables; just ensure we end up saying the same thing with the same configuration, without being hampered by the way autotools are defining things).