Oh, you meant for me to reply =)

- Do all unix-like system support pkg-config?

Yes. It even works on windows.

- Is $(LIBDIR)/pkgconfig the only choice for installing this file?

While pkg-config can be told to look in other directories for pc files
(via modifying the PKGCONFIGPATH env variable) it looks in
/usr/lib/pkgconfig by default, so that's where pc files should be put by
default.

- Shouldn't this file be installed only if the pkg-config utility is
present, or if the directory $(LIBDIR)/pkgconfig already exists?

Technically I suppose so, but I've not seen any package actually bother.

The patches I have made available are very similar to pkg-config
packages I've seen for many other packages.

cheers,