After squashing most of our compiler warnings, up'ed the default
warning level from 3 to 4 in order to increase the likelyhood of
catching future warnings.
Defined the DEBUGBUILD pre-processor variable to allow extra logging,
which is particularly useful in debug builds, as we use this and Visual
Studio typically uses _DEBUG.
We could define DEBUBBUILD, in curl_setup.h, when _MSC_VER and _DEBUG is
defined but that would also affect the makefile based builds which we
probably don't want to do.
Carrying on from commit 11025613b9 added a more thorough version of
the VC6 project files which are capable of supporting side-by-side
compilation, as well as support for some of the third-party libraries
curl uses.
