Fix building on OSX again

In file included from selectmplexer.cc:6:
In file included from ./syncres.hh:22:
In file included from ./mtasker.hh:26:
/usr/include/ucontext.h:43:2: error: The deprecated ucontext routines require _XOPEN_SOURCE to be defined
 ^
In file included from selectmplexer.cc:6:
In file included from ./syncres.hh:22:
In file included from ./mtasker.hh:113:
./mtasker.cc:272:3: error: use of undeclared identifier 'getcontext'
  getcontext(uc);
  ^
2 errors generated.
make[4]: *** [selectmplexer.o] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2