Log Message:
www/firefox68: Resolve pkg-build-options/not-bl3 issue

firefox68 tries to use pkg-build-options to find out if gtk3 was (or
will be) built with wayland, as that affects the PLIST.  The current
code works for some and causes failures for others, including failures
of thunderbird.  pkg-build-options insists on only being called from
bl3, but the use in Makefile (to manage PLIST changes) seems sensible.

This commit removes the use of pkg-build-options, resolving the build
issues on netbsd-8, and adds a default-off wayland option to firefox68
that merely adjusts the PLIST, so that people building firefox68 with
a wayland-enabled gtk3 have an easier time.

I don't believe that any default-option binary packages will change,
so no PKGREVISION++.

A proper fix is deferred until after the branch.  This could involve
allowing pkg-build-options to be used in Makefile* instead of only
bl3, or adding wayland detection and setting some variable to gtk3's
bl3.

As discussed on pkgsrc-users and offlist with nia@.