By 2030 change default setting to WITH_LIBDIVIDE=OFF in CMakeLists.txt

[kimwalisch]

We currently use libdivide because integer division is very slow on most x64 CPUs. However integer division will likely become much faster in near future and then WITH_LIBDIVIDE=OFF is expected to improve primecount's performance. But carefully benchmark some large inputs e.g. 1e21 --AC -s.

[kimwalisch]

libdivide is now disabled by default (since primecount-7.11) for Apple Silicon CPUs on Apple OSes in CMakeLists.txt.