pkgs/cmake doesn’t build cleanly when e.g. ncurses version changes

[stapelberg]

When moving from ncurses-amd64-6.1-4 to ncurses-amd64-6.1-5, I got:

make[2]: *** No rule to make target '/ro/ncurses-amd64-6.1-5/out/lib/libncurses.so', needed by 'bin/ccmake'.  Stop.

Needed to remove the builddir before I could get it to work.