pkg-autotools: add support to gettextize a package

In case a package comes with a gettext infra which is different from
the one Buildroot provides, we'd get autoreconf errors, like:
    http://autobuild.buildroot.net/results/c0f/c0f7c801f61fdc310cde64342060b00a70155431/



To avoid that, we need to gettextize the package prior to running
autoreconf.

Provide the necessary infrastructure in the autotools infrastructure,
so we can do it consistently across packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>