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/bitcoinrpc.cpp

@@ -1418,7 +1418,7 @@ Value listsinceblock(const Array& params, bool fHelp)
         CBlockIndex *block;
         for (block = pindexBest;
              block && block->nHeight > target_height;
-             block = block->pprev);
+             block = block->pprev)  { }
 
         lastblock = block ? block->GetBlockHash() : 0;
     }