Bug #1160802:  Can't build Zope on Windows w/ 2.4.1c1.

MSVCCompiler.initialize():  set self.initialized to True, as suggested
by AMK.  Else we keep growing the PATH endlessly, with each new C
extension built, until putenv() complains.

No change to NEWS because the patch that created this bug is also new
for 2.5a1 (so there's no change here to any code yet released from HEAD).