versioning C++ includes

[Benjamin Kosnik]

Currently, g++/libstdc++ installs files in:

(prefix)/include/g++-v3

It's done this for some time. Although this does work, there are a
couple of problems with the current approach:

1) People who think they are using gcc-3 and libstdc++-v3 because of
   the '-v3' parts of the above install directory in older compilers,
   including gcc-2.96, gcc-2.95.x, etc. This has been a constant,
   low-level annoyance for years.

2) Adding and especially removing C++ include files in a common
   (prefix) directory results in versioning problems, aptly summarized
   by the __basic_file problems that were reported my multiple people
   during the gcc-3.0.x to gcc-3.1.x upgrade process.

3) At this point, the libstdc++.so binary is versioned. With similarly
   versioned headers, the library ABI could be fully controlled, and a
   lot of the patches that have been queued by Phil and Paolo could go
   in.

Because of this, I consider the status quo something that can be improved.

Here's my suggestion:

Future g++/libstdc++ installs files in:

(prefix)/include/c++/VERSION

Where version is the release number. 

This matches java's clearly named language-orientated installation
directories, and the 

(prefix)/lib/gcc-lib/(target)/VERSION/include

bits from gcc.

Thoughts? I'll start working on implementing the above, and wait for
additional input.

-benjamin