app: g_file_new_for_uri() return object not freed with g_object_unref().

Non-freed GFile object when saving or exporting the first time, i.e.
with default extensions.
app: set win32 only code between #ifdef G_OS_WIN32

PATHEXT environment variable to determine whether a file is executable
or not is a Windows-only concept.
configure: fix testing for bzip2 on windows