[MERGE] [0.90] document optional pyrex dependency in INSTALL

[Sabin Iacob]

Paul Moore wrote:
> """
> bzr can optionally use compiled versions of some parts of the code for
> increased speed. When installing bzr you need the ability to build C
> extensions. Some Linux distributions package the necessary headers
> separately from the main Python package - if yours is one of these,
> look for a (probably optional) package called something like
> python-dev. FreeBSD and Windows users have all the necessary headers
> installed by default.
> """
>
> Paul.
>   

maybe it's worth mentioning that rpm-based distros use something like 
"python-devel" and deb-based distros use "python-dev"; also, 
source-based distros (like Gentoo + derivatives and possibly Slackware + 
derivatives) can be added to *BSD and Windows