Make grab compile on OS X.

Move the CPU affinity stuff and MAP_POPULATE into ifdef blocks so they
only are used on Linux.

Remove a couple of unused class variables that the compiler was warning
about. They were both redefined in function locals anyway.

You'll still need to provide your own PCRE since one does not come with
OS X. I compiled against the one in pkgsrc, but that required changing
the Makefile to include -I and -L flags to use my unprivileged
installation.