Unfortunately the Make-lang.in part is still needed when using the --disable-libada configure switch so a more elaborate change is needed.
> Remove the extraneous `install-gnatlib' invocation from within gcc/ then > as all the gnatlib handling ought to be done in libada/ nowadays. > > gcc/ada/ > * gcc-interface/Make-lang.in (ada.install-common): Remove > `install-gnatlib' invocation.
