bug#43725: 28.0.50; Include feature/native-comp into master

[Lars Ingebrigtsen]

I haven't gotten very far in reviewing the patch set (because it's very
large), and I don't have any comments so far (because it all looks very
good :-)).  Just two minor comments on building and running:

If libgccjit isn't installed, configure says this:

checking for dlfunc... no
configure: error: Installed libgccjit has failed passing the smoke test.
You can verify it yourself compiling:
<https://gcc.gnu.org/onlinedocs/jit/intro/tutorial01.html>.
Please report the issue to your distribution.
Here instructions on how to compile and install libgccjit from source:
<https://gcc.gnu.org/wiki/JIT>.

Instead of saying that it isn't installed.

Also -- starting Emacs says "Compilation started.", and it seems like
it'll say that now and then when using Emacs, too.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no