Add a customizable variable for driver options (such as linker flags)
to pass to libgccjit (Bug #42761).

* lisp/emacs-lisp/comp.el (comp-native-driver-options): New
customization variable.
* src/comp.c: Use comp-native-driver-options to set libgccjit's driver
options, if supported on the library's ABI version.

Ensure the variable is set to the value that was customized in the
parent process in child compilation processes, also.

Otherwise, this would break async compilation with a merely
semi-visible error message.

* Instead of conditionalizing on the wrong preprocessor flag, now use
  the right one: LIBGCCJIT_HAVE_gcc_jit_context_add_driver_option

* Also perform the driver-option-adding step on win NT, but only if
  the function is non-NULL.

* Make the function declaration for add_driver_options non-old-style.

Resolves merge conflict in comp.el