[build] Fix bootstrap/51072: libitm not disabled without c++

[Gerald Pfeifer]

On Sat, 17 Dec 2011, Eric Botcazou wrote:
>> Putting into gcc/cp/config-lang.in is a layering violation, it's true.  But
>> until there's another instance that needs handling, it seems premature to
>> build infrastructure to handle this.  And it's only one line after all...
> I don't know exactly why, but this breaks bootstrap on some machines:

I may be wrong, but it occurs to me your log already shows why?

> make[3]: Entering directory 
> `/red.a/gnatmail-x/build-red/x86_64-linux/gnat/obj/libcpp'
> g++  -I../../src/libcpp -I. -I../../src/libcpp/../include ...
> cc1plus: error: unrecognized command line option "-Wno-narrowing"

Here g++ is invoked without an absolute path, so if you have an older 
version of g++ in your path that does not grok -Wno-narrowing yet that 
will fail, won't it?  I assume that's why only some like you and me are 
seeing this, but not all of us.

Gerald