nir_intrinsics.c existed as a static file until commit 76dfed8a began
generating it as part of the build process. autotools is incapable of
coping, and so a build-tree from before this commit would then fail with
it:

[4]: *** No rule to make target '../../../mesa/src/compiler/nir/nir_intrinsics.c', needed by 'nir/nir_intrinsics.lo'.  Stop.

Add a few lines to configure.ac to update the broken build files.

Fixes: 76dfed8a ("nir: mako all the intrinsics")