configure.ac: supersede --enable-gallium-llvm over --enable-llvm

configure.ac: supersede --enable-gallium-llvm over --enable-llvm

Currently we have extra (somewhat questionable) modularity, such that
one could build some parts with LLVM while others w/o.

That is extremely fragile, error prone and requires quite noticable
amount of code throughout.

Thus lets deprecate the gallium toggle in faviour of the generic one.
The former will throw a warning when set, and it will be overwritten by
the latter. This will allow gradual transition w/o breaking people's
scripts.

v2: Rebase, document in release notes.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de> (v1)