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

[Eli Zaretskii]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: Eli Zaretskii <eliz@gnu.org>, 46983@debbugs.gnu.org
> Date: Sun, 07 Mar 2021 19:35:01 +0000
> 
> Okay dbdc44db15 should allow for `comp-native-driver-options' as
> file-local to be captured correctly.

Thanks.

> 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?

I'd say, if this is set in file-local variables, just ignore the
setting if it cannot be applied.  Is this possible to ignore it in
file-local vars, but still signal an error if customized?