[PATCH GPGME] python: build: reinstate prepare target

[Alon Bar-Lev]

BTW:

there is abnormality in the build system that the sources are modified
if --build-base is specified, the following files are written to
source instead to build directory or build temp directory:
      errors.i
      gpgme.h
      gpgme_wrap.c
      gpg/gpgme.py

Please consider to avoid.

Doing so will enable us to prepare a single source and then build
using separate build base using multiple python versions. This is
actually how downstream builds.

Regards,
Alon