os/mac/pkgconfig: add libffi.pc

[Bo98]

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[Bo98]

Should mean that some things that depended on brewed libffi can now depend on system libffi.

Though there's the question of whether it's worth it - we'd have to deal with a prehistoric version 2.1 on Mojave and earlier (where we likely need brewed libffi instead for some formulae) while having a modern 3.3-rc0 on Catalina (where the system library should be fine). Would be awkward to have this everywhere:

if MacOS.version < :catalina
  depends_on "libffi"
else
  uses_from_macos "libffi"
end

[MikeMcQuaid]

Would be awkward to have this everywhere:

We could extend uses_from_macos to support before: and/or after: arguments. That was in the original PR, didn't work and so I removed it when fixing other issues.

[MikeMcQuaid]

Thanks @Bo98!