Fix POSIX compliance

* "WAIT_MYPGRP" isn't POSIX'ish. Relying on this caused compilation
  issues in certain environments (e.g. under Cygwin). As a workaround,
  define "WAIT_MYPGRP" explicitly if it's undefined.

* "P_tmpdir" is an XSI extension. Don't try use it if it isn't
  available.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/utils.c

@@ -523,8 +523,10 @@ get_tempdir (void)
 {
   if (getenv ("TMPDIR"))
     return getenv ("TMPDIR");
+#ifdef P_tmpdir
   else if (P_tmpdir)
     return P_tmpdir;
+#endif
   else
     return "/tmp";
 }