Refactored the evn override handling to a single method and fixed the handling

Refactored the evn override handling to a single method and fixed the handling
of the case where ENV is not a dictionary which could happen when the
environment variable ENV was set when invoking SCons.

Fixed building dynamic library on Windows in the case where env overrides was
specified as before these was not passed to the linking of the DLL.

There is still a SCons issue when the environment variable ENV is set when
invoking SCons, however this looks like a SCons issue.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@156 ce2b1a6d-e550-0410-aec6-3dcde31c8c00