build: Allow libportal support to be disabled

It isn't clear whether the API/ABI of libportal are entirely stable yet (https://github.com/flatpak/libportal/issues/33) so it is not necessarily appropriate for longer-term-supported OS distributions to include it. When building a version of epiphany for a distribution package, which is only intended to be packaged in a format other than as a Flatpak app, libportal isn't necessary anyway.

libportal is also Linux-specific, so non-Linux OSs will likely want to disable it (even if it might compile successfully).