Fix cross build.

The cross build fails to find the lopsub library since it configures
for the build architecture. This happens as no --build nor --host is
passed which would have happened automatically if dh_auto_configure
could be used. Thus it'll have to be passed manually. Then configure
fails finding config.h.in, which for some reason is not created (nor
asked for) by the override_dh_autoreconf. And finally, the actual
Makefile does not pick up the compiler detected by configure and
rather uses plain cc, so we'll have to tell it as well.

Signed-off-by: Andre Noll <maan@tuebingen.mpg.de>