Now that we use the native compiler, and have fixed Qt, and these vars are (almost) unset in Guix, we can remove the unsetting from our compiler command here.
I couldn’t manage to make a darwin-clang-cross only exclusion of -lm
work properly
for Qt, so opted for just removing the explicit link entirely. I do not think this should have
any other unwanted side-effects.
Fixes #21552.