BUILD: fix kFreeBSD build.

BUILD: fix kFreeBSD build.

kFreeBSD needs to be treated as a distinct target from FreeBSD
since the underlying system libc is the GNU one. Thus, relying
only on __GLIBC__ no longer suffice.

- freebsd-glibc new target, key difference is including crypt.h
  and linking to libdl like linux.
- cpu affinity available but the api is still the FreeBSD's.
- enabling auxiliary data access only for Linux.

Patch based on preliminary work done by @bigon.

closes #1555