Revert and extend previous change pin check change

Reviert previous change of #if BLAH_PIN > 0 to #if defined(BLAH_PIN) &&
BLAH_PIN > -1. Unfortunately some times pin 0 is used. For my sins I've
gone through and replaced all unsafe checks of #if BLAH_PIN > -1 with
the safe version.

Marlin/Marlin.pde

@@ -47,6 +47,6 @@
   #endif
 #endif
 
-#if DIGIPOTSS_PIN > -1
+#if defined(DIGIPOTSS_PIN) && DIGIPOTSS_PIN > -1
 #include <SPI.h>
 #endif