Add DOCS option to net-im/libsignon-glib.
This makes the documentation optional (but defaults to on). Without
the documentation, no gtkdoc needs to be built, and no Doxygen gets
The ports framework for --enable-<foo> doesn't help here, because that
option is created by the gtkdoc autoconf macros, which we don't want
to have involved. Use sed to comment out the offending bits. Patch
considerably changed from the PR.
PORTREVISION bumped because there's now options where there were
none before, but the default package doesn't change at all.
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