gettext: force build libintl if needed

This commit introduces a hidden option
BR2_PACKAGE_GETTEXT_PROVIDES_INTL that is used by gettext.mk to force
the build of libintl if the toolchain doesn't provide a full gettext
implementation.

For now, this hidden option is not selected by anything, but a
follow-up commit will introduce BR2_ENABLE_NLS, which will make use of
it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>