mingw: do not trust MSYS2's MinGW gettext.sh

It does not quite work because it produces DOS line endings which the
shell does not like at all.

This lets t0200-gettext-basic.sh, t0204-gettext-reencode-sanity.sh,
t3406-rebase-message.sh, t3903-stash.sh, t7400-submodule-basic.sh,
t7401-submodule-summary.sh, t7406-submodule-update.sh and
t7407-submodule-foreach.sh pass in Git for Windows' SDK.

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

Author: dscho

config.mak.uname

@@ -561,6 +561,7 @@ else
 		INTERNAL_QSORT = YesPlease
 		HAVE_LIBCHARSET_H = YesPlease
 		NO_GETTEXT =
+		USE_GETTEXT_SCHEME = fallthrough
 		USE_LIBPCRE= YesPlease
 		NO_CURL =
 		USE_NED_ALLOCATOR = YesPlease