Get rid of underscore prefixes for include guards.

This is because they may conflict with reserved names. Also took
this opportunity to make the guards in the zcash subdir a bit
more uniform by having them all conform to ZC_<...>_H_

This closes #1272

src/zcash/JoinSplit.hpp

@@ -1,5 +1,5 @@
-#ifndef _ZCJOINSPLIT_H_
-#define _ZCJOINSPLIT_H_
+#ifndef ZC_JOINSPLIT_H_
+#define ZC_JOINSPLIT_H_
 
 #include "Zcash.h"
 #include "Proof.hpp"
@@ -102,4 +102,4 @@ protected:
 typedef libzcash::JoinSplit<ZC_NUM_JS_INPUTS,
                             ZC_NUM_JS_OUTPUTS> ZCJoinSplit;
 
-#endif // _ZCJOINSPLIT_H_
+#endif // ZC_JOINSPLIT_H_