respect $PKG_CONFIG

We don't want to hardcode `pkg-config` because that breaks cross-compiling
that supplies its own pkg-config wrapper.  At least respect the PKG_CONFIG
env var like the upstream pkg-config m4 intends.

Really this code should be using PKG_PROG_PKG_CONFIG, but the brltty seems
to be avoiding standard tools like aclocal and autoheader and automake.