util/Error: use FormatMessage() instead of g_win32_error_message()
system/FatalError: use FormatMessage() instead of g_win32_error_message()
system/FatalError: add FatalSystemError() overload with WIN32 error code
Win32Main: use FatalSystemError()
configure.ac: make GLib optional by default

Only very few (minor) features still depend on GLib.
configure.ac: require GLib 2.32 (if enabled)

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.