autoconf: Initial support for an autoconf configuration

autoconf: Initial support for an autoconf configuration

This adds the initial support for using autoconf configuration. Support
is available for shared Xlib driver builds. Later this will be extended
to dri and osmesa-only builds and possibly targetting non-X backends.
Support for static library builds will also be added.

The configure script fills in the autoconf config. This is then used by
running `make autoconf' after ./configure.

Testing has been done on Linux/GNU. The configure script tries to
faithfully reproduce the current configs/linux* and configs/freebsd*.
Other platforms can be handled later by adding similar statements and
feature tests.

Pkg-config is used to search for packages when possible. This makes the
build much more flexible and robust to the user's configuration. This
requires that the pkg-config autoconf macros pkg.m4 are included in
aclocal.m4. This requires autoconf and aclocal from autoconf and
automake, respectively.