RFA: Another installed-tree testing patch

[Benjamin Kosnik]

> This is another patch for installed-tree testing using
> v3_target_compile_as_c.  The directories we're searching for C++ files
> (when using the C compiler) need to include
> $target/include/c++/$version, for a cross compiler.
> 
> As I've noted in the comment, this is all an ugly hack; the right
> answer is for the compiler to have a -print-include-dirs option that
> we could pass to g++, and then just include that list for the C
> compiler.  But, we don't have that.

Yes. We've been around about this before, remember?

http://gcc.gnu.org/ml/gcc/2006-10/msg00328.html

> Tested with a cross to arm-eabi.  OK to apply?

These both look great.

-benjamin