Clean up th ecompile process by centralizing the include files

- code compile tested, but due to a yet unresolved problem with
          parse.h's creation, compile not completed...

src/include/commands/explain.h

@@ -0,0 +1,17 @@
+/*-------------------------------------------------------------------------
+ *
+ * explain.h--
+ *    prototypes for explain.c
+ *
+ * Copyright (c) 1994-5, Regents of the University of California
+ *
+ * $Id: explain.h,v 1.1 1996/08/28 07:21:47 scrappy Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef	EXPLAIN_H
+#define	EXPLAIN_H
+
+extern void ExplainQuery(Query *query, List *options, CommandDest dest);
+
+#endif	/* EXPLAIN_H*/