The skip-checking is now in the functions for adding plugins and
subdirectories, so that third-party building should get it
as well, for free. Since AddModuleSubdirectory and AddPlugin
use the newly split-out module, handling SKIP_MODULES and USE_*
consistently across module repositories is now easier.
While here, make accumulating-the-skipped-modules explicit.
- both clang and g++ support __builtin_unreachable(); (as Kevin
Kofler pointed out) so we don't need the macro to do different things;
- the compilers have gotten better at detecting unreachable code,
so instead of inserting macros or fiddly bits, just drop them
and the unreachable code they comment.
- reduce the difference between clang and g++ builds, factor
common flags out of the CMake-if
- drop special boost-warning-suppression, we do that differently
most of the time in the affected source files
- point to main Calamares site in the 'part of' headers instead
of to github (this is the "this file is part of Calamares"
opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
this is the 3-paragraph summary of the GPL-3.0-or-later, which has
a meaning entirely covered by the SPDX tag.
The build instructions are not that interesting, it's a toss-up
between CC0 and BSD-2, but because other CMake bits are BSD-2-Clause,
apply that to more CMakeLists. The copyright date isn't all that
accurate, but these are just inconsequential files.
While here, tidy up and get rid of some useless intermediates.
- like Esperanto before Qt 5.12, Interlingue does not
seem to be supported by QLocale, so it gets turned into
"C" locale, which then messes up the default language
selection in the welcome page.
Move it to _incomplete until QLocale does support it.
FIXES#1475
