wscript: mark subprocess as required

We either need to be on Windows, or have posix_spawn available.

If someone can come up with a system that is POSIX, but does not provide
posix_spawn, we could make it optional.

wscript

@@ -227,6 +227,7 @@ iconv support use --disable-iconv.",
         'desc': 'posix_spawnp() or MinGW',
         'func': check_true,
         'deps_any': ['posix-spawn', 'mingw'],
+        'req': True,
     }, {
         'name': 'glob',
         'desc': 'glob()',