Reduce compiler warnings on Windows and MacOSX

Reduce compiler warnings on Windows and MacOSX

DCHECK* causes "warning C4127: conditional expression is constant".

If compiled with -DWIN32_LEAN_AND_MEAN,
port.h: warning C4005: 'WIN32_LEAN_AND_MEAN': macro redefinition.

On MacOSX,
warning: "_END_GOOGLE_NAMESPACE_" redefined
warning: "_START_GOOGLE_NAMESPACE_" redefined
because config.h is included
 from base/commandlineflags.h:51
 from logging.cc:60
and
 from utilities.h:81
 from logging.cc:32
or so.

Contributed by ukai@

git-svn-id: https://google-glog.googlecode.com/svn/trunk@114 eb4d4688-79bd-11dd-afb4-1d65580434c0