poppler: fix configure by pointing on moc

As discussed in [1] configure was detecting host's moc and failing in some
configurations.

To get this fixed configure gets a parameter by which the location of moc
is set.

[1] http://lists.openembedded.org/pipermail/openembedded-devel/2015-May/101641.html



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>