build: better POSIX check

This is the "official" method to do this.

wscript

@@ -156,9 +156,7 @@ main_dependencies = [
     }, {
         'name': 'posix',
         'desc': 'POSIX environment',
-        # This should be good enough.
-        'func': check_statement(['poll.h', 'unistd.h', 'sys/mman.h'],
-            'struct pollfd pfd; poll(&pfd, 1, 0); fork(); int f[2]; pipe(f); munmap(f,0)'),
+        'func': check_statement(['unistd.h'], 'long x = _POSIX_VERSION'),
     }, {
         'name': '--android',
         'desc': 'Android environment',