[v3] fixup --enable-cxx-flags

[Oleg Endo]

On Fri, 2012-09-28 at 21:12 -0700, Benjamin De Kosnik wrote:
> ... found while working on arm-eabisim cross, using --enable-cxx-flags
> was not working as AM_CXXFLAGS was being over-ridden by CXXFLAGS.
> (Despite the comments warning about this.)
> 
> Fixed.
> 
> Also patchlet for the last commit, forgot to edit PARALELL_FLAGS, so
> --disable-thread compiles were failing. 
> 
> -benjamin
> 
> tested x86/linux
> tested x86/linux --enable-cxx-flags="-g0"

Hm, this doesn't help PR 53579, does it?

Cheers,
Oleg