Compile with extra warnings turned on. And more makefile/code tidying up.

This turns on most gcc warnings, and removes some unused variables and other code that triggers warnings.
Exceptions are:
 -Wno-sign-compare : triggered by lots of comparisons of signed integer to foo.size(), which is unsigned.
 -Wno-char-subscripts : triggered by the convert-to-hex functions (I may fix this in a future commit).

src/headers.h

@@ -79,8 +79,6 @@
 #endif
 
 
-#pragma hdrstop
-
 #include "serialize.h"
 #include "uint256.h"
 #include "util.h"