cross-compiling and python

[Tor Lillqvist]

> --enable-build-python=<system/internal>

But we have a generic --with-build-platform-configure-options option
that can be used to pass several options to the sub-configury for the
build platform. Would it work to use:

--with-build-platform-configure-options=--enable-python=internal
--enable-whatever-else --with-something ?

We do have some leftover --with-system-foo-for-build options there but
they are obsolete. At least a comment says so. The generic
--with-build-platform-configure-options should hopefully be enough.

--tml