plugin headers no longer installed?

[Jack Howarth]

   Is the following issue appearing on any other targets? On x86_64-apple-darwin10,
sometime after r164502 but by r164564, the installation of plugin headers was lost.
At r164502, we still had the previous behavior on darwin where plugin headers were
installed in...

/sw/lib/gcc4.6/lib/gcc/x86_64-apple-darwin10.5.0/4.6.0/plugin/include

when FSF gcc was built with --prefix=/sw/lib/gcc4.6. At r164564, this directory is
no longer being installed. This will prevent the use of gcc trunk for building the
dragon-egg plugin with the installed headers.
           Jack