dbus-c++: xml2cpp allow caller to add arbitrary macros

Allow the caller to define arbitrary macros which will be used when
expanding the template file.  This allows for easy customization of
the output.

BUG=none
TEST=none

Change-Id: I3a03f095b7d3e9b7e6432add3ddb50d340a56928
Reviewed-on: https://gerrit.chromium.org/gerrit/17742
Tested-by: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Eric Shienbrood <ers@chromium.org>
Commit-Ready: Jason Glasgow <jglasgow@chromium.org>

• tools/generate_stubs.cpp[diff]
• tools/generate_stubs.h[diff]
• tools/xml2cpp.cpp[diff]