Replace mkdtemp() with _ev_g_mkdtemp().

The function mkdtemp() is not available on Windows. Unfortunately,
glib does not have a portable replacement for it yet, see
http://bugzilla.gnome.org/show_bug.cgi?id=524831. This patch copies
a proposed implementation for g_mkdtemp() from that bug
report (renamed to _ev_g_mkdtemp()). Fixes bgo#604917.