When we reconfigure, `configure_file()` is called again, and
`*.gresource.xml` files are regenerated, which causes many (all?)
binaries to be relinked. Now we only write those out if the contents
actually changed (or if the output didn't already exist).
This is exactly what Meson already does with `configure_file()` when
`command:` is not used.
While we're at it, also do the same for `gen-c-array.py` and
`gentypefuncs.py` for completeness. Now even if the input to those
changes, re-building of those custom targets may not result in
relinking if the outputted C files have the same contents.
After commit 2ab9be54fb we had to rename
the generated CSS files to be included into the GResource bundle; we
kept the URI stable to avoid too much churn, and allow backporting the
change to the 3.24 stable branch. This had the adverse effect of making
it harder to debug issues, as the on-disk file name does not match the
location in the GResource that will be used to print out warnings,
errors, and debugging messages.
It seems that Meson's gnome.compile_resources() cannot deal with two
files with the same name under different directories, which breaks the
build parallelism because the GResource file ends up not depending on
either the Adwaita or the HighContrast gtk-contained.css file.
This commit only changes the on-disk names of the Adwaita and
HighContrast SCSS files, and the corresponding generated CSS files; the
files in the GResource are going to be aliased to the old file names, to
minimise the breakage. We might want to change the theme entry points at
some later date, if we decide to commit to this naming scheme.
Fixes: #2423
See Meson bug: https://github.com/mesonbuild/meson/issues/6615
