python: Unbreak Python third-party extensions

This patch fixes compilation/linking of python third-party extensions, i.e.
Extensions that ship with C code.

Problem:
Python uses distutils(-native) to compile third-party extensions. distutils
uses its own sysconfig module to get the options for compiling and linking.
Since third-party extensions have to be linked against this libpython it
important that -L points into staging. This is not the case because
distutils.sysconfig uses a special Makefile that is shipped with python
determine the paths. The Makefile is the same that would be used on the
target to build third-party extensions. It therefore points into /usr/lib
instead of staging.

Solution:
Stage a modified version of the Makefile where the paths (incdir, libdir) have
been replaced by ones that point into staging.

Side-problem:
The recipe actually should not stage files itself in do_compile, but rather
handle everything that needs to be staged in do_install. This is currently not
possible because python compiles itself using distutils-native. Distutils on
the other hand does only allow to add a path, but not to substitute it,
requiring a staged Makefile and libpython.so before the actual python
compilation is triggered.

The second step to solve this would be to either patch distutils, or split
python into python-initial and python. The -initial part could create the
Makefile and the library, while the main part focuses on the target.

For further references see:
 http://lists.linuxtogo.org/pipermail/openembedded-core/2011-May/001752.html

Signed-off-by: Michael Lippautz <michael.lippautz@gmail.com>
Acked-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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