poppler: help finding libtiff in static linking scenarios

In static linking scenarios, poppler doesn't find libtiff because it
doesn't use pkg-config, so it doesn't know which additional libraries
it should link with to test the availability of libtiff. We help
poppler in this work by passing a LIBTIFF_LIBS variable, resulting for
a pkg-config invocation.

Also, since poppler uses pkg-config, we make this dependency explicit
in this patch, even though it was already carried by the fontconfig
dependency.

Fixes:

  http://autobuild.buildroot.org/results/be7/be78957dd852233c81bd364ddf664564ce8f0208/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>