BUG(2470): Protect HAVE_* values in environment.
BUG(2470): Protect HAVE_* values in environment.

In waf 1.6 the write_config_header method that writes the
defines to a xmms_configuration.h file also resets all the
defines in the environment to 0. This is fixed by overriding
the undefine method so that the HAVE_* variables keep their
original values.

chrippa/xmms2,theeternalsw0rd/xmms2,xmms2/xmms2-stable,xmms2/xmms2-stable,theeternalsw0rd/xmms2,krad-radio/xmms2-krad,theefer/xmms2,theefer/xmms2,xmms2/xmms2-stable,krad-radio/xmms2-krad,theefer/xmms2,chrippa/xmms2,krad-radio/xmms2-krad,chrippa/xmms2,chrippa/xmms2,krad-radio/xmms2-krad,six600110/xmms2,theeternalsw0rd/xmms2,theefer/xmms2,xmms2/xmms2-stable,six600110/xmms2,six600110/xmms2,theefer/xmms2,krad-radio/xmms2-krad,theeternalsw0rd/xmms2,six600110/xmms2,xmms2/xmms2-stable,theeternalsw0rd/xmms2,theefer/xmms2,chrippa/xmms2,xmms2/xmms2-stable,six600110/xmms2,theeternalsw0rd/xmms2,chrippa/xmms2,krad-radio/xmms2-krad,theefer/xmms2,six600110/xmms2