[CFT] add a config.site cache for the ports

[Warren Block]

On Mon, 18 Mar 2013, Baptiste Daroussin wrote:

> Hi,
>
> The autotools allows us to have a config.site cache where we define our defaults
> values for a couple of things, and prevent the "slow" and possibly wrong
> autodetection.
>
> Here is a patch that makes use of it:
> http://people.freebsd.org/~bapt/autotools_config_site.diff
>
> As the libiconv/gettext update has shown the configure scripts can fall back on
> gnu version of commands first if it find it, and in case gettext is removed you
> can get trouble.
>
> In this config.site, I hardcoded a couple of FreeBSD binaries in order to always
> use them, but I let the toolchain being autodetected.
>
> I also added a couple of headers to avoid useless checks and more can be added
> in the futur.
>
> Any thought?

Avoiding the repeated detection of the exact same things could seriously 
reduce build times for big ports.  Thank you!