speedo: Add appimage targets for Speedo builds
* build-aux/speedo.mk (appimage, git-appimage, this-appimage): New targets. (help): Add documentation. (define SETVARS, define SETVARS_W64): Set environment variables for cross-compiling only if TARGETOS is w32.
The new appimage targets allow building those parts of GnuPG that are
needed for creating an AppImage of gpg.
Differences to a native build are:
- gpgme is not built
- pinentry-qt is built
- documentation of gnupg is not built
- GnuPG-bug-id: T5598