Without the change the build on `gcc-11` fails as:
$ nix build --impure --expr 'with import ./. {}; qt4.override { stdenv = gcc11Stdenv; }' -L
...
messagemodel.cpp: In function 'int calcMergeScore(const DataModel*, const DataModel*)':
messagemodel.cpp:186:61: error: ordered comparison of pointer with integer zero ('MessageItem*' and 'int')
186 | if (c->findMessage(m->text(), m->comment()) >= 0)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
2d4606a32d