bug#46983: 28.0.50; [feature/native] Allow .el files to specify non-default values for native-compilation options

[Andrea Corallo via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Andrea Corallo via "Bug reports for GNU Emacs, the Swiss army knife of
text editors" <bug-gnu-emacs@gnu.org> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>> This is a feature request.  Since there are various stability and
>> reliability issues with libgccjit, some Emacs Lisp files may need to
>> specify non-default values for some native-compilation options, such as
>> 'comp-speed' or 'comp-native-driver-options'.
>
> Hi Eli,
>
> if the interface we'd like to expose is the through file-local variables
> then this should work already for `comp-speed' and `comp-debug',
> I'extend it for `comp-native-driver-options'.

Okay dbdc44db15 should allow for `comp-native-driver-options' as
file-local to be captured correctly.

But...

I've only added 'gcc_jit_context_add_driver_option' to GCC 9 so we have
to decide what to do when this is not available on older GCC.

Till today we raised an error as the use of `comp-native-driver-options'
was a deliberate user choice and it's effectiveness was testable with
`comp-native-driver-options-effective-p', but if this becomes part of
source code we should probably transition to a warining? WDYT?

Thanks

  Andrea