Make stdenv handling less intrusive.

* Use "-I" instead of "-isystem" in $CPPFLAGS, as the latter is only
  supported in GCC/Clang, while the ENVVAR is used in other situations
  and may break building.
* Don't encode "-Qunused-arguments", as this is only supported in Clang
  and will break building when C/C++ compiler is switched during the
  process, e.g. building gcc. The warning is harmless without
  "-Werror".