Win32: manage gmp installation with mingw-get
Win32: keep mingw package versions more or less in alphabetical order
Win32: manage pexports installation with mingw-get
Win32: cleanup inst_gnome step for new mingw environment

- gettext and friends are provided by mingw, drop gnome versions
- fix "library not found" issue for pkg-config
- pkg-config no longer needs a wrapper to avoid weird output
- reorganize some installation steps
Win32: adapt inst_guile step to new mingw environment
Win32: fix libofx build for new mingw environment
Win32: fix libdbi build in new mingw environment
Win32: update isocodes package version
Win32: add code to extraxt XZ compressed archives

Patch by Gary Bilkus