[v3] add -Wabi to build flags

[Benjamin Kosnik]

As requested by Jason, I've added -Wabi to the libstdc++ build flags.
The patch is trivial. The results are in line with what we expect.

After I did this part and verified the build went swimmingly, I ran the
testsuite with -Wabi as well. This part had some problems, I'm getting
some odd results (things like constexpr functions being marked as
changing with -fabi-version=3, which makes no sense to me.)

I've filed this stuff in bugzilla with some notes, see the link for
more info:
http://gcc.gnu.org/PR51804

As an aside, the mangling changes for -fabi-version=3 look like they
will impact a lot of code.

tested x86/linux

-benjamin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 20120109-1.patch
Type: text/x-patch
Size: 870 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20120109/45c81ab6/attachment.bin>