[plugins] RFA: Merge plugins into mainline [4/5 - configury]

[Paolo Bonzini]

I don't see a reason for putting the PLUGINLIBS code in the toplevel
rather than in gcc/.  This would cause problems if I modify
gcc/configure.ac, run autoconf, and then run make from within gcc/.

I think this belongs in a macro in config/plugins.m4 (ACX_LIBS_PLUGINS
or something similar) or even in gcc/acinclude.m4 since I don't
foresee many other projects in src using the plugin infrastructure.

This further change is pretty trivial, so I'll preapprove a variant of
the patch that drops changes in the toplevel.

Thanks,

Paolo