Update bootstrap script to work from separate repository
Win32: some improvement to the bootstrap script
Win32: introduce a wrapper function around mingw-get

This function is to work around mingw-get's unfortunate refusal to
upgrade a package when calling mingw-get install on an already
installed one or mingw-get upgrade on a package that's not installed
Win32: manage msys installation via mingw-get
Win32: manage mingw installation via mingw-get
Win32: manage autotools installation via mingw-get
Disable updating mingw-get's package catalog for now

That allows me to manually correct a couple of packaging bugs
while waiting for an upstream fix
Win32: manage gettext installation via mingw-get
gettext should be before binutils
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: add flag to guile configure to fix broken test for regcomp_rx
Win32: fix guile's strncasecmp test on mingw
Win32: fix guile's srfi module detection
Win32: fix libofx build for new mingw environment
Win32: fix libdbi build in new mingw environment
Win32: update isocodes package version