Qt & gcc requirements for 2.4

[Pavel Sanda]

Hi,

since (at least) fff28c57561a our master does not compile with gcc 4.9 anymore.
Also it seems that there is agreement that Qt4 won't be supported for 2.4 either.
What should we state in the INSTALL now?

I guess that boils down to two separate questions:

- minimal (theoretical) requirements:
  I see JMarc dropped  __cplusplus >= 201402L, what it means in term of gcc(clang)?
  Is there someone running Qt < 5.15?


- real tested combinations:
  What are the real Qt numbers/gcc(/..) numbers we use?


I mostly run gcc 10.2.1 & Qt 5.15.2 but largely still depend on 2.3.6, so
it would be worth to known what ppl really running 2.4 use...


Pavel

PS: I still keep the old Qt4/gcc 4.9 combo to solely check there were are
    2.3.x compliant, but will drop it immediately after 2.3 final release.