I too encountered issues compiling QMC2 r8335. It compiles against Qt 4.8.7 as it had done before, but it then crashes at any attempts to launch mame systems and doesn't display some of the submenu contents. Compiling against Qt 5.9.5 and 5.15 are unsuccessful, yielding:
Project ERROR: Unknown module(s) in QT: xmlpatterns webkit webkitwidgets
Makefile:1063: recipe for target 'Makefile.qmake' failed
I believe those modules do exist though, particularly in 5.15 as it's a very recent Qt build.
Using Linux Mint 19.3 (Ubuntu 18.04 base), with gcc 9.3.0 and kernel 5.3.
The last time it successfully compiled was against r8292 (from late 2018), though I hadn't tried again untli now.
Rene (or anyone), any ideas what could be wrong here?