`newsboat` contains quite a bit of C++ code built by custom Makefile.
`rust` code and tests already get build in parallel. The change enables
build parallelism for custom Makefile as well.
The effect on my machine is 78% build time reduction (4.5x speedup):
Before the change:
$ time nix build -f. newsboat --rebuild
real 6m20,904s
user 0m0,345s
sys 0m0,167s
After the change:
$ time nix build -f. newsboat
...
real 1m23,592s
user 0m0,277s
sys 0m0,097s
ef91c7bcd8