gettext: allow building when BR2_NEEDS_GETTEXT is not selected

Until now, we thought that gettext was only needed for the target for
uClibc toolchains, to provide the gettext functions that are normally
provided directly by glibc.

However, the gettext runtime actually does more than providing the
equivalent of those C library functions: it also provides certain
command line tools, like 'gettext' to get translated strings from the
shell. This tool is for example used by certain ecryptfs-utils scripts
to get translated strings. It is therefore necessary to be able to
build the gettext package even for glibc toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>