Lyx-2.3.1 install under Linux-Mint

[Wolfgang Engelmann]

I wanted to install the recent Lyx-2.3.1 under Linux-Mint. In the Quick 
compilation guide it says
-----------------------

These four steps will compile, test and install LyX:
    0) Linux users beware: You need qt4/5 and qt4/5-devel packages
       of the same version to compile LyX

     ....

However, I don't find qt5 under Mint (synaptic or apt-get install). 
Could one of you, who uses Mint and Lyx-2.3.1 could tell me how I get it?

Wolfgang