distutils can't handle paths to source files containing '..'. It will
try to navigate away from the build directory and fail. To fix that,
before building the python module, transform all the path variables then
cd to the srcdir, and set the build directory manually.

This is hopefully the last needed fix to use separate build and
source diretories.
Signed-off-by: Daniel Miranda <danielkza2@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>