BUG: Change working dir when building extension.

Current working directory should be changed (to trusted one) when building
an extension so compiler accidentally won't pick up anything from (untrusted)
current directory.

Ran scipy.weave tests and they passed:

    $ scipy.weave.test('full', verbose=10)

    Ran 443 tests in 147.585s

    OK (KNOWNFAIL=2)


This is extremely minor security issue which should not supposedly be able to be exploited.