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)

docs/releasing.html

@@ -354,7 +354,7 @@ Here is one solution that I've been using.
 		--with-gallium-drivers=swrast \
 		--with-vulkan-drivers=intel \
 		--enable-llvm-shared-libs \
-		--enable-gallium-llvm \
+		--enable-llvm \
 		--enable-glx-tls \
 		--enable-gbm \
 		--enable-egl \

docs/relnotes/17.1.0.html

@@ -55,6 +55,7 @@ Note: some of the new features are only available with certain drivers.
 <h2>Changes</h2>
 
 <li>Removed the ilo gallium driver.</li>
+<li>The configure option --enable-gallium-llvm is superseded by --enable-llvm.</li>
 
 </div>
 </body>