Use _SC_NPROCESSORS_ONLN for CPU detection in BSDs.

[tritao]

Fixes #2328.

[nickclark2016]

Do you have a FreeBSD VM or something where we can test the build?

[tritao]

Do you have a FreeBSD VM or something where we can test the build?

I don't and don't really have the disk space at the moment available to install one. I'm fairly sure this will be ok as unistd.h is already included in BSDs and this API is available on BSDs.

@Martinfx Would you be willing to confirm this patch works?

[Martinfx]

Do you have a FreeBSD VM or something where we can test the build?

yes. I have FreeBSD on desktop.

[Martinfx]

Do you have a FreeBSD VM or something where we can test the build?

I don't and don't really have the disk space at the moment available to install one. I'm fairly sure this will be ok as unistd.h is already included in BSDs and this API is available on BSDs.

@Martinfx Would you be willing to confirm this patch works?

Thank you. It works.