Compute flags for target in configure, not Makefile

[Jason Merrill]

This seems wrong to me; the $(build_tooldir)/lib and
$(build_tooldir)/include stuff should be used iff we aren't using
newlib, but your patch causes it to always be used.  Why can't the
build_tooldir stuff all be part of FLAGS_FOR_TARGET?

Also, I'm not sure what the compiler will do with -isystem flags when
compiling a CHILL file.

Jason