swig: avoid host contamination, make build deterministic

* Explicitly disable all other languages than python, which was the
  only language listed in DEPENDS.

* Inherit pythonnative to make it find OE's python executable.

* Depend on libpcre in order to reliably enable Swig's regex feature.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>