Update exim.Makefile to v4.97 from v4.96

It seems like that when the exim package was updated to v4.97 the build configuration file wasn't updated to the corresponding version as well, this PR merges the new config with Arch's older one and doesn't touch any previous option.

For reference the v4.97 upstream build conf file can be found here: https://git.exim.org/exim.git/blob/107077d7fd6736711bf5cd980221723401d37c51:/src/src/EDITME

I did test that Exim builds and just executes with this patch (no real world testing tho).

( btw, thank you Antonio Rojas very much for picking up the package from the orphanage :3 )