meson.build: add pkgconfig variable for portal support

Portal support is optional in libei, yet EI clients cannot easily tell whether support for the portal was enabled in libei at build time.

Add a pkgconfig variable 'ei_portal' that reflects whether or not libei was built with portal support, so that clients can automatically enable the same if available.

Signed-off-by: Olivier Fourdan ofourdan@redhat.com