php: 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. Thanks to this, we can drop
   the BR2_USE_WCHAR dependency

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