Qt5 port group

[Vincent Habchi]

Sorry to chime in back so late, I had several hiccups with my mail lately, and most (if not all) of your messages simply remained in transit until today.

As far as I have experimented, a single version of Qt is allowed to be installed. Fair enough.

I’ve rewritten the Qgis3 port introducing a +qt59 variant. I think what can be done could be something like:

Variant qt59 conflicts … {
	depends-lib-append	qt59- (list of libraries)
}

using explicit qtXX- dependencies.

Or, if we want to spare space:

Variant qtXX conflicts qtYY … {
	set qt_version XX
}
…
Depends-lib_append qt${qt_version}-… 

The port group could be modified to check variants are compatible with MacOS X versions, so e.g. requesting Qt 5.9 on Snow Leopard would result in an error.

Vincent