SIGILL on older x86_64 hardware

[orlitzky]

I have an older x86_64 (Core 2 Duo) laptop that lacks the popcnt instruction. The check in CMakeLists.txt erroneously adds -mpopcnt to its compiler flags, and that eventually causes Illegal Instruction (SIGILL) crashes.

IIRC popcnt can be detected at runtime, and certainly at build time with a compile/run test. But it may be simpler these days to just trust that most people have -march=native in their CXXFLAGS when compiling locally.

[galabovaa]

Thank you, this is a bit that's been needing revision but it got left further back on the to do list