Export XSI sigpause

Export XSI sigpause

There are two common sigpause variants, both of which take an int argument.
If you request _XOPEN_SOURCE or _GNU_SOURCE, you get the System V version,
which removes the given signal from the process's signal mask; otherwise
you get the BSD version, which sets the process's signal mask to the given
value.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>