autoconf: add --enable-{dri,glx,osmesa}

The idea is that DRI driver, libGL and libOSMesa are libraries that can
be independently enabled, yet --with-driver does not allow us to easily
do that, if not impossible.  This also matches what
--enable-{egl,xorg,d3d1x} do for the respective libraries.

There are two libGL providers: Xlib-based and DRI-based.  They cannot
coexist.  To be able to choose between them, --enable-xlib-glx is also
added.

With this commit, --with-driver=dri can be replaced by

  $ ./configure --enable-dri --enable-glx --disable-osmesa

--with-driver=xlib can be replaced by

  $ ./configure --disable-dri --enable-glx --enable-osmesa \
                --enable-xlib-glx

and --with-driver=osmesa can be replaced by

  $ ./configure --disable-dri --disable-glx --enable-osmesa

Some combinations that cannot be supported with --with-driver will
produce errors at the moment.  But in the future, we would like to
support, for example,

  $ ./configure --enable-dri --disable-glx --enable-egl
  (build libEGL and DRI drivers, but not libGL)

Note that this commit still keeps --with-driver for transitional
purpose.

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