export more plugin headers

[Frank Ch. Eigler]

Diego Novillo <dnovillo@google.com> writes:

> 2009/6/22 Ralf Wildenhues <Ralf.Wildenhues@gmx.de>:
>
>> This patch does not ensure that the set of files installed remains
>> sufficient for compiling plugins against it.
>
> Nor can it (though it would be nice if we had such mechanism).  [...]

You could include a sample demo plugin source file in the gcc install
tree, and as a part of the "make install" command, build-test it only
against those installed plugin headers.  If the gcc testsuite is able
to do an "installcheck" (install-tree test run), it could test this
too.

- FChE