The default C++ headers install place

[Dave Korn]

Mark Mitchell wrote:
> Ian Lance Taylor wrote:
> 
> [adding gcc-patches]
> 
>> @item --with-gxx-include-dir=@var{dirname}
>> Specify
>> the installation directory for G++ header files.  The default is
>> @file{@var{prefix}/include/c++/@var{version}}.
>>
>> and that is incorrect.  Personally, I would be fine with something like
>> "the default depends upon how gcc was configured."
> 
> I agree.
> 
> For the benefit of gcc-patches, it was pointed out on gcc-help that the
> description of the default directory used by --with-gxx-include-dir is
> incorrect.  The actual default is computed by a complex bit of configury
> code, so stating it clearly -- let alone keeping it up to date -- in the
> manual is hard.  If you don't care, use the default; if you do care, you
> ought not to rely on the default.

  Thanks for doing this.  As a concrete example, Cygwin is a fairly mainstream
platform that doesn't install the gxx headers into $prefix/include, we leave
them in the compiler-private dir $prefix/lib/gcc/$target/$version/include/c++,
which is the default you get when --enable-version-specific-runtime-libs is in
play.

    cheers,
      DaveK