configure.ac: Improve pkg-config related checks.

configure.ac: Improve pkg-config related checks.

Until now, we checked for certain (optional) libraries via pkg-config and
the configure script would abort if any of them was not found, even
though they were optional. It was up to the user to then figure out which
combination of --disable-<drivername> switches were required for his
specific OS (and set of installed libs) to get a working configure run.

Only if the user already specified enough --disable-<drivername>
switches beforehand, so that all drivers which require a missing library
were disabled, would the configure run not check for that specific lib
(and would thus not fail).

With this change, we now always unconditionally check for all libs
(required and optional) via pkg-config. However, whether an (optional) lib
is found or not, configure will not abort. Instead, it'll just disable
all drivers which need a lib that cannot be found.

The user will no longer have to supply --disable-<drivername> parameters
in order to get a working build.