meson: Add option to skip cargo-build target

This is only needed when building docs or similar
to avoid overhead.

meson_options.txt

@@ -8,3 +8,7 @@ option(
   value: 'default',
   description: 'The build profile for Fractal. One of "default" or "development".'
 )
+option(
+  'skip-cargo-build', type: 'boolean', value: false,
+  description: 'Skip building "cargo-build" target.'
+)

src/meson.build

@@ -52,7 +52,7 @@ sources = files(
 
 custom_target(
   'cargo-build',
-  build_by_default: true,
+  build_by_default: not get_option('skip-cargo-build'),
   input: sources,
   output: meson.project_name(),
   console: true,