C signed right shift portability

[Kevin Ryde]

This isn't a function as such, but it's a bit of a C trap, albeit a
rare one.

        * autoconf.texi (Function Portability): Mention C right shifts.

The gcc sources suggests it always does an arithmetic shift, so if
someone can confirm that then perhaps add:

        GCC however, as of version 3.0, generates arithmetic shifts on
        all processors it supports.

autoconf.texi.rshift.diff
Description: Source code patch