[v3] RFC libstdc++11convenience.la

[Benjamin Kosnik]

This breaks separates the libstdc++ source files for C++11 from the
C++03 sources, and puts C++11 files in a new source directory,
src-c++11, which is compiled into libstdc++11convenience.a, and linked
in to libstdc++ in the same manner that libsupc++ uses.

Thus, no change to user-level or tool-level linking. To use C++11,
you'd still just link in libstdc++. To use C++03, you'd still just link
in libstdc++. 

The build rules for C++11 were established early on. This seems like a
useful abstraction now that so many of the sources have to be built
with C++11 flags. 

At some point, the continued ad hoc nature of
src/Makefile.am's current approach will break down.

At some other point, we might want to start building a libstdc++11
library with different flags from src/c++03. This is how to get there.

tested x86/linux

-benjamin
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