Commit 10575666 was too eager and did
not consider SLES 12 which still has 2.0.1 that does not ship
a pkg-config file.

Similar to how we check for readline, prefer pkg-config if available
and fall back to the old detection code if not found.

NB: this is not a clean revert because we're not reintroducing support
for YAJL 1.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reported-by: NOlaf Hering <olaf@aepfle.de>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>