Clean/fix up the Preprocessor Definitions for the various projects, where
we purge out the unneeded macros and add _DEBUG to the Debug builds of
various projects that somehow lacked this.
This will also fix the GIO build under Visual C++ 2008, as the _DEBUG macro
in the release builds will cause a debug entry to appear in its manifest
file during the build, which will cause GIO-using applications to fail
to run on systems not running Visual C++/Studio 2008 due to its embedding
of a badly-generated manifest file.