gettext: rationalise optional dependencies

gettext has optional dependencies on libxml2, glib, libcroco and libunistring.
If they're not available then gettext will use internal copies, but it can also
use system libraries.

For gettext-native and nativesdk-gettext continue to use the internal copies to
reduce the dependencies, but for target use the system shared libraries.

Also gettext 0.19.7 onwards swapped expat for libxm2, so remove the build
dependency on expat.

(From OE-Core rev: a82aa376a9229428ec25629e97a1efa56f0afae7)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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