portconfigure.tcl: Always sanitize cxxflags
portconfigure.tcl: Always sanitize cxxflags

Correctly handle the case where configure.cxx_stdlib is cleared after
configure.cxxflags is read.

This means that Portfile authors can't put "-stdlib" into
configure.cxxflags themselves, as it'll get scrubbed. They shouldn't be
doing this anyway.

git-svn-id: 620571fa9b4bd0cbce9a0cf901e91ef896adbf27@126358 d073be05-634f-4543-b044-5fe20cf6d1d6

macports/macports-base,macports/macports-base,danchr/macports-base,danchr/macports-base,neverpanic/macports-base,neverpanic/macports-base