Get rid of use of dlltool in Mingw builds.

We are almost completely out of the dlltool game, if this works.

Hiroshi Inoue
Flush a stray definition of $(DLLTOOL).

Even if this is needed, it'd be configure's responsibility to set it.
Make gendef.pl emit DATA annotations for global variables.

This should make the MSVC build act more like builds for other platforms,
i.e. backend global variables will be automatically available to loadable
libraries without need for explicit PGDLLIMPORT marking.

Craig Ringer