stop clobbering CXXFLAGS

We have CXXFLAGS already setup.  No configure script should ever
clobber or override the way this one does.

BUG=chromium-os:39738
TEST=`emerge-x86-alex dbus-c++` uses our CXXFLAGS

Change-Id: Ied463631267f72e44311f41b6946abe282c7709d
Reviewed-on: https://gerrit.chromium.org/gerrit/45036
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>