configure: Redirect pkg-config errors to log file

If pkg-config doesn't find a library it prints an error to stderr.
Instead of directly printing them as part of the configure output,
redirect them to config.log and let the user know about it in case there
are any messages.

Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>