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/sigs.c

@@ -43,6 +43,10 @@
 
 #include "calcurse.h"
 
+#ifndef WAIT_MYPGRP
+#define WAIT_MYPGRP 0
+#endif
+
 /*
  * General signal handling routine.
  * Catch return values from children (user-defined notification commands).