mingw: let's use gettext with MSYS2

mingw: let's use gettext with MSYS2

This solves two problems:

- we now have proper localisation even on Windows

- we sidestep the infamous "BUG: your vsnprintf is broken (returned -1)"
  message when running "git init" (which otherwise prevents the entire
  test suite from running) because libintl.h overrides vsnprintf() with
  libintl_vsnprintf() [*1*]

The latter issue is rather crucial, as *no* test passes in Git for
Windows without this fix.

Footnote *1*: gettext_git=http://git.savannah.gnu.org/cgit/gettext.git
$gettext_git/tree/gettext-runtime/intl/libgnuintl.in.h#n380

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>