bug#40969: 28.0.50; [feature/native-comp] unnecessary recompilation

[Christian Thäter]

I notice that when I 'org-reload' C-c C-x ! then the whole
compilation starts again even when its already compiled. Somewhere it
misses that. I didn't investigate this any further. Maybe its intended
behavior because org-mode forces to byte-compile itself.

Might be smarter to store a hash over the source .el or .elc in the .eln
metadata and prevent recompilation when it was already compiled. In fact
all .eln could be addressed by these hashes with some directory
restructuring (I dont go into detail here, this would be another
feature).

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo 
version 1.16.0)
 of 2020-04-26 built on wolke
Repository revision: e6fb440ed7e48c2e8c4bba4666db2f0ff9950a25
Repository branch: native-comp
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)

Configured using:
 'configure --with-nativecomp --with-cairo
 --enable-link-time-optimization --with-xwidgets'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY ACL
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS XWIDGETS PDUMPER
GMP

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix