Improve the speedo make script.

* scripts/gpg-w32-dev/README: Remove
* scripts/gpg-w32-dev/GNUmakefile, scripts/gpg-w32-dev/speedo.mk:
Merge into ..
* scripts/speedo.mk: this.

Speedo may now be used to build from GIT or tarballs and has a couple
of other improvements as well. For best results the autogen.sh files
of all source package should be updated to the one used in gnupg.

• Signed-off-by: Werner Koch <wk@gnupg.org>