Use PKG_INSTALLDIR to determine pkg-config file install location

The PKG_INSTALLDIR macro also enables --with-pkgconfigdir for easy
overrides.

PKG_INSTALLDIR is relatively new, it appeared in pkg-config 0.27. To
ensure we have a recent pkg.m4 copy, copy pkg.m4 to the m4 directory.

configure.ac

@@ -255,8 +255,7 @@ sysconfdir="/etc";
 localstatedir="/var";
 fi
 
-pkgconfigdir=${libdir}/pkgconfig
-AC_SUBST(pkgconfigdir)
+PKG_INSTALLDIR
 
 AC_CONFIG_FILES([
   common/Makefile