Move sass compilation to its own custom target

[SparkyBluefang]

run_command runs silently and the build log does not indicate that the command was run, or what the command was, so it's not great for build debugging. On the other hand, custom_target logs as a normal build target.

[9/113] /usr/bin/pysassc ../cinnamon-6.4.3/data/theme/cinnamon-sass/cinnamon.scss data/theme/cinnamon-sass/cinnamon.css

run_command also states that it runs in an unspecified directory, so the current script is relying on undocumented implementation/platform specific behavior. Rather than updating the script to ensure it runs in the proper directory, custom_target does this for us (and handles installs since it enforces the source/build separation).

Also support both pysassc and sassc (which was the reason I was in here in the first place X_X). The only difference between the two is some decimal precision.

--- a/data/theme/cinnamon-sass/cinnamon.css (pysassc)
+++ b/data/theme/cinnamon-sass/cinnamon.css (sassc)
@@ -994,13 +994,13 @@ StScrollBar StButton#hhandle {
     spacing: 12px; }
   .workspace-switch-osd-indicator {
     background-color: rgba(255, 255, 255, 0.5);
-    padding: 2.66667px;
-    margin: 13.33333px;
+    padding: 2.6666666667px;
+    margin: 13.3333333333px;
     border-radius: 32px; }
     .workspace-switch-osd-indicator:active {
       background-color: #3584e4;
-      padding: 5.33333px;
-      margin: 10.66667px; }
+      padding: 5.3333333333px;
+      margin: 10.6666666667px; }
 
 .monitor-label {
   border-radius: 0;

Theoretically this could also add dart-sass ('sass') to the list of programs, but that has more differences (like outputting rgb(x, y, z) values instead of hex, and over 100 deprecation warnings). While [py]sassc/libsass are deprecated, I'm not advocating for a migration to dart-sass (yet) since Gentoo doesn't package dart, and I'm not signing up to maintain that.

[leigh123linux]

sassc is a dead project.

https://github.com/sass/sassc

[SparkyBluefang]

Yes, I'm aware, and libsass-python is (functionally) equally dead since libsass is dead, thus the footnote about dart-sass. Given packaging consideration, I'd rather use sassc since we don't need the python bindings (which seems like the only reason to choose libsass-python).

But if that's going to be a sticking point, I can remove sassc, since the run_command/custom_target changes still have merit IMO.