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/checkpoints.cpp

@@ -52,7 +52,6 @@ namespace Checkpoints
     {
         if (fTestNet) return NULL;
 
-        int64 nResult;
         BOOST_REVERSE_FOREACH(const MapCheckpoints::value_type& i, mapCheckpoints)
         {
             const uint256& hash = i.second;