build: allow some python variable overrides

build: allow some python variable overrides

The python variables (settings) are fetched from a running python
interpreter which usually isn't the target one when cross compiling,
hence libraries and flags aren't the same and can pollute the target build.
Allow some of these variables to be redefined via environment variables
in order to aid cross-compiling.
According to testing python_LDFLAGS and python_LIBDIR should be enough.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Feb 13 06:03:26 CET 2014 on sn-devel-104