Create and install a nix-util.pc

Before, `-lnixutil` was just stuck in `nix-store.pc`, but that doesn't
seem so nice.

This prepares us to distribute `libnixutil` in a separate package if we
want, but it should be a good change either way. I suspect it wasn't
done before because libutil was an extra unstable interface, but I don't
think we need worry about that. *All* the C++ is less stable than the C
(or that's the goal at least).

For what it's worth, Lix also created this pkg-config file *en passant*
during their rename:
c97e17144e (diff-3c4f60cc44a0e35444c7f45331cfa50f76637118)

src/libutil/local.mk

@@ -40,3 +40,5 @@ $(foreach i, $(wildcard $(d)/signature/*.hh), \
 ifeq ($(HAVE_LIBCPUID), 1)
   libutil_LDFLAGS += -lcpuid
 endif
+
+$(eval $(call install-file-in, $(buildprefix)$(d)/nix-util.pc, $(libdir)/pkgconfig, 0644))