pdfium now has a build_config.h which is something between a generated
file (it's not part of git) and a normal header (works in a way, so the
same header works for all platforms), handle it the same way as clucene.

Change-Id: I674eca0c52fb8681c3a20501e39f95a8f9d8724e
Reviewed-on: https://gerrit.libreoffice.org/50840Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>