ExternalProject usually involve a configure and a make
step that produce a bunch of output usually irrelevant
including a large number of warning and other mess.
now that everything is pretty much in tail_build
these output get interleaved with useful output from
the build of the product and actually drown them in a logorrhea
of messy noise.
This store the output of external modules in a log file
and only print them as a whole if the module failed do build.
on a non-verbose build.

Change-Id: I3abfcccd6d16821a9e061a71e031b427cc283647
Reviewed-on: https://gerrit.libreoffice.org/2304Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>