support multiple versions of C++ headers

[Jeff Johnson]

When we uplift C++ headers for things like Qt and libstdc++, we will probably
need to keep old versions of those headers as well to support older versions of
the LSB. Thus, we will need some mechanism to support this.

Alternatively, we could support C++ header generation directly from the
database (which could then provide the usual #ifdefs to do the right thing).
But that's not likely for the near future.