[MERGE] Better warnings when pyrex is not available

[Vincent Ladeuil]

>>>>> "john" == John Arbash Meinel <john at arbash-meinel.com> writes:

    john> Since have finally merged our first Pyrex extension into bzr, we have now
    john> started running into the user difficulties.

Don't forget dev difficulties ;-)

I ran into it yesterday, pyrex should be installed, but python-dev
should be installed too or the compilation failed.

       Vincent