Using qt59 instead of qt5 (latest release)

[Craig Treleaven]

> On Apr 4, 2018, at 9:54 AM, Vincent Habchi <vince at macports.org> wrote:
> 
> Hey Craig,
> 
>> I am considering a similar move for mythtv.28.  
> 
> […]
> 
> Thanks for that useful info. French has a saying which goes like this (more or less, it’s difficult to translate): “The best is oftentimes the good’s enemy”.
> In any case, yeah, I suppose this will mean at least writing variants. What I am more weary of is py-qt5. I hope the latest version is backwards compatible with earlier releases of Qt5.
> 
> Probably going to do that later in the week.
> 

I was wondering if the Qt5 portgroup could be modified so that dependent ports don’t automatically upgrade to the absolute latest version of Qt 5.  Just spitballing, but what if the portgroup had a directive like:

prefer_QT5_version     -1

If the currently released version is 5.10, this would indicate that we want to use 5.9.  When 5.11 is released, presumably 5.10 would be mature enough that it would be safe to upgrade to it.

Or perhaps:

prefer_Qt5_LTS_version [-1]

Qt 5.9 is the current long term support version and Qt 5.6 is the previous LTS.  

Thoughts?  Are most Qt5-dependent ports happy to keep up to the latest Qt release?  It appears to be a concern for Vincent and myself.

Craig