[LyX/master] Cmake build: Start demanding at least version 5.1 if compiling with gcc

[Pavel Sanda]

On Fri, Oct 25, 2024 at 12:43:26AM +0200, Jean-Marc Lasgouttes wrote:
> >I thought that distros which have GCC<8 do not ship qt5, but qt4.
> >Am I wrong here? Otherwise how are you going even to test it?
>
> Ubuntu 15.10 is GCC 5.4 and Qt 5.4. Just what we said except for python.
> 
> I can almost compile LyX 2.4 with it, 

Arrgh, packages.ubuntu.com betrayed me. It's not that Qt 5/ Python 3
is not available, it just does not search through older than 20.04
because these are on extended security support only.

Googling around 
16.04 has Qt 5.5.1 & Python 3.5 & gcc 5.4 available even if not default.
18.04 has Qt 5.9.5 & Python 3.6 & gcc 7.5 available

We did unnecessarily drastic cut. 
We can require gcc 7 only, but I think you just killed support Qt 5.9.5 anyway?

Pavel