configure: Silence pkg-config calls

Redirect the output of pkg-config to /dev/null so it doesn't clutter the
output of the configure script.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
configure, Makefile: Fail in Makefile if Config is not complete

Define CONFIG_OK in the Config file generated in the configure script
and check for it being defined in the Makefile before attempting
anything else. This way we can better inform the user that she needs to
run ./configure first.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>