Make use of the standard macro for PKG_CONFIG; the default behaviour is now
to use Pango if present, instead of requiring explicit user request, yet
still not making it mandatory.
The detection code was moved to a macro to keep the configure.ac script
(relatively) small, and consistent with what is done for most other libs.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
f049635fa5