gettext: optimize build time

This commit significantly reduces the build time of host-gettext and
gettext, by using the capacity of gettext to handle build things in a
certain subdirectory:

 - For the host variant of gettext, we only need the gettext-tools,
   available in the directory of the same name in the gettext sources.

 - For the target variant of gettext, we only need the gettext library
   libintl, available in the gettext-runtime directory in the gettext
   sources.

So by using appropriate values of GETTEXT_SUBDIR and
HOST_GETTEXT_SUBDIR, we only build what's necessary. Moreover, by
manually patching gettext-tools/Makefile.in and
gettext-runtime/Makefile.in, we make sure to not build and install
things like examples, documentation and so on.

In addition to this, these changes avoid the need to autoreconfigure
the gettext package, which was particularly long.

Thanks to these changes, the build time of gettext goes from 1 minutes
and 37 seconds to just 24 seconds, and the build of host-gettext goes
from 2 minutes and 18 seconds to 1 minute and 13 seconds.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>