This target doesn't build on macOS (even with the upstream make based
build system) so this encodes that in the build without requiring
non-linux users to disable it manually with the starlark flag. The flag
is still respected for linux users.
We used to have `pfm` built into exegesis, although since it's an external dependency we marked it as a manual target. Because of this we didn't have buildbot coverage and so we removed it in D134510 after we had a few breakages that weren't caught. This adds it back, but with three possible states similar to the story with `mpfr`, i.e. it can either be disabled, built from external sources (git/make), or use whatever `-lpfm` is installed on the system.
This change is modeled after D119547. Like that patch, the default is off (matching the status quo), but unlike that patch we don't enable it for CI because IIRC we don't have the package installed there, and building from source might be expensive. We could enable it later either after installing it on buildbot machines or by measuring build cost and deeming it OK.
Reviewed By: GMNGeoffrey
Differential Revision: https://reviews.llvm.org/D138470
