PATCH: silence build warning from libstdc++

[Benjamin Kosnik]

> When building libstdc++, I see (paths trimmed for brevity):
> 
> stdtr1c++.h:29: warning:
> include/powerpc-linux/bits/stdc++.h.gch/O2ggnu++0x.gch: not used
> because `__GXX_EXPERIMENTAL_CXX0X__' not defined

I don't see this on x86_64-linux. 

But, it looks valid to me just the same. Can you tell me how to
reproduce? I would be surprised if this is the first you've seen of
this, as the previous stdtr1c++.h include also had two choices for
stdc++.h precompiled files.

> I propose the following patch to fix this (unless anyone can suggest a
> better way?).  Okay for mainline?

Better would be to just edit include/Makefile.am to remove
-Winvalid-pch from PCHFLAGS.

-benjamin