qt5 build dependency on clang

[Ryan Schmidt]

> On Apr 22, 2017, at 15:55, db <iamsudo at gmail.com> wrote:
> 
> On 22 Apr 2017, at 21:58, Ryan Schmidt <ryandesign at macports.org> wrote:
>> Sounds plausible. What's your question / problem with this?
> 
> Ken already point me to cxx11-1.1.tcl, but I still don't get why 4.0 isn't whitelisted in a system whose default is set to 3.9.

I guess clang 4.0 was not released at the time that the list of supported compilers in portconfigure.tcl in MacPorts base was last updated. It is released now, so we can certainly add it now, and that change would then be in the next version of MacPorts base.