build: restore support for libyajl 2.0.1

build: restore support for libyajl 2.0.1

Commit 105756660f944e7db02de3b55b98bb7c11cd03bf 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: Ján Tomko <jtomko@redhat.com>
Reported-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>