[PATCH modular] Per-component configure options

[Trevor Woerner]

I'm surprised nobody seems interested in this patch, I would have
thought a general mechanism to provide per-component configuration
options would have been a feature for which people would have been
waiting; especially the distribution maintainers.

I was trying to perform a build from the git repositories on a clean
installation of a Ubuntu 10.04.3 (LTS) machine and found that I
couldn't build mesa/mesa successfully without supplying the
"--enable-nouveau-experimental-api" configuration option when building
mesa/drm. On my openSuSE 12.1 machine (which I normally use) the build
of mesa/mesa gets contaminated by the system headers (which I hadn't
noticed at the time) which are provided by the libdrm-devel package.
The Ubuntu 10.04.3 machine doesn't have this header in /usr/include so
the build fails. In any case a git build shouldn't be contaminated in
this way.

Ideally it would be great to perform a sysroot-style build at some
point to find out what other modules are being contaminated by system
include files, but that will have to wait until I find the motivation
and/or time.