configure.ac: Add required LLVM versions to the top

Consolidate the required LLVM versions at the top where the other
versions for dependencies are listed.

v5:
Splitted out separate changes (see patch 19 and 20)

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed