Swap the condition -- instead of checking for OS X and assuming anything else uses the Linux behavior, check for Linux and let other platforms use the default POSIX behavior. This allows sigaltstack to be used on BSD variants.
This change has been tested (with the standard test suite) on FreeBSD 9.1 (i386, amd64) and Ubuntu 12.04.1 (i386).
Remove #ifdefs so ss_flags is always set to zero.

Removed the #ifdefs which used a different value for 'ss_flags' on Linux.
According to the POSIX standard, the SS_ONSTACK flag can never be used as input to the 'sigaltstack' function -- it can only be set by the 'sigaltstack' function itself, and tested after the function returns.
Merge pull request #555 from jack-pappas/sigaltstack-patch

Fix sigaltstack on BSD