Keep on churning.

Keep on churning.

Without #define _XOPEN_SOURCE macports clang 9.0.1 on OSX 10.7.5 was
giving me

In file included from swapcontext.c:12:
/usr/include/ucontext.h:43:2: error: The deprecated ucontext routines require
      _XOPEN_SOURCE to be defined
 ^
swapcontext.

So I added #define _XOPEN_SOURCE

But that gives, on Solaris 11.3

In file included from /usr/include/limits.h:12:0,
                 from /usr/gcc/4.8/lib/gcc/i386-pc-solaris2.11/4.8.2/include-fixed/limits.h:168,
                 from /usr/gcc/4.8/lib/gcc/i386-pc-solaris2.11/4.8.2/include-fixed/syslimits.h:7,
                 from /usr/gcc/4.8/lib/gcc/i386-pc-solaris2.11/4.8.2/include-fixed/limits.h:34,
                 from swapcontext.c:7:
/usr/include/sys/feature_tests.h:354:2: error: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications  and pre-2001 POSIX applications"
 #error "Compiler or options invalid for pre-UNIX 03 X/Open applications \
  ^

So make the #define _XOPEN_SOURCE conditional on darwin.