Some license identifiers are ambiguous (e.g. "GPL3"). The SPDX license
identifiers avoid this by providing standardized and unique identifiers
(e.g. "GPL-3.0-only" or "GPL-3.0-or-later" for the previous example).

Because SPDX short-form identifiers are also both human- and
machine-readable we should recommend them in the documentation.

More information (advantages, details, etc.) can be found here:
- https://spdx.dev/resources/use/#identifiers
- https://spdx.dev/ids/

Fix #7905.

Using the std option, so now `rust_std=..` will work. I've chosen to use
"std" even though rust calls these "editions", as meson refers to
language versions as "standards", which makes meson feel more uniform,
and be less surprising.

Fixes: #5100

Right now sub-sub projects are not correctly registered, because we
don't have a way to pass up past the first level of subproject. This
patch changes that by making the build_Def_files as defined in the
Interpreter initializer accurate for translated dependencies, ie, cmake
dependencies won't define a dependency on a non-existent meson.build.
This means that it can always add the subi.build_def_files because they
are always accurate.

Subprojects that use the CMake PCH feature will cause
compilation/linker errors. The CMake PCH support
should thus be disabled until this can be properly
translated to meson.

* depenencies/llvm: Handle llvm-config --shared-mode failing

Fixes: #7371
Fixes: #7878

* test cases/llvm: Refactor to use test.json

Instead of trying to cover everything internally

When glib is a subproject we should use glib-compile-resources it
overrides using find_program() in the case it is not installed on the
build machine. With old glib version we have to run
glib-compile-resources at configure time to generate the list of
dependencies, but not when glib is recent enough.

This is regression test for #7573