Jakub found an error in my earlier patch.  If a gnu_inline function
was redeclared without a new definition, and then called, we'd ICE
when inlining it.  Here's a patch that fixes it, by arranging for the
linkage flags to not be affected by mere redeclarations, only by
actual redefinitions.

Jakub offered to test it, so I haven't, other than by compiling by
hand the test included in the patch.  Ok to install after Jakub
completes testing it?

:ADDPATCH c++/gnu_inline: