Having a BR2_NEEDS_GETTEXT option, as introduced by
54d64798e1 isn't sufficient to express
the different kind of dependencies on gettext.
This commit, based on an idea by Peter Korsgaard, introduces two
different options :
* BR2_NEEDS_GETTEXT, which is true as soon as the toolchain doesn't
provide gettext itself (i.e, when the toolchain is uClibc based, be
it an internal or external toolchain)
* BR2_NEEDS_GETTEXT_IF_LOCALE, which is true when the toolchain
doesn't provide gettext *and* locale support has been enabled in
Buildroot.
A following commit adds some documentation that details how these
configuration variables should be used by packages.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>