configure.ac: factor out detection for old and buggy llvm

As of LLVM 3.9 one could use consistent ways to handle the component.
Factor out the current handling, as it will be used for older versions.

Cc: mesa-stable@lists.freedesktop.org
Cc: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
1 file changed