buildroot

libvips: use the new gettext logic

This commit switches to use the new gettext logic, which involves:

 - using TARGET_NLS_DEPENDENCIES instead of hand-encoded dependencies
   on gettext/host-gettext

 - dropping BR2_PACKAGE_GETTEXT selection. We adjust the comment
   explaining the BR2_USE_WCHAR dependency, since gettext is no longer
   the reason for this dependency.

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