configure: fix to make older pkg-config play well

configure.ac:1349: error: possibly undefined macro: PKG_CONFIG_LIBDIR

Obviously this is not a problem with pkg-config 0.26 but older versions
seem to show this.

Fix suggested by: Kamil Dudka
Reported by: Guenter
Bug: http://curl.haxx.se/mail/lib-2011-11/0298.html