build: Replace deprecated build_root() and source_root() calls

Also use variables since we call these functions multiple times
- might as well cache them since they stay the same the whole
time.

src/meson.build

@@ -4,16 +4,19 @@ configure_file(
     configuration: conf
 )
 
+build_root = meson.project_build_root()
+source_root = meson.project_source_root()
+
 # Copy the config.rs output to the source directory.
 run_command(
   'cp',
-  meson.build_root() / 'src' / 'config.rs',
-  meson.source_root() / 'src' / 'config.rs',
+  build_root / 'src' / 'config.rs',
+  source_root / 'src' / 'config.rs',
   check: true
 )
 
-cargo_options = [ '--manifest-path', meson.source_root() / 'Cargo.toml' ]
-cargo_options += [ '--target-dir', meson.build_root() / 'src' ]
+cargo_options = [ '--manifest-path', source_root / 'Cargo.toml' ]
+cargo_options += [ '--target-dir', build_root / 'src' ]
 
 if get_option('profile') == 'default'
   cargo_options += [ '--release' ]
@@ -24,7 +27,7 @@ else
   message('Building in debug mode')
 endif
 
-cargo_env = [ 'CARGO_HOME=' + meson.build_root() / 'cargo-home' ]
+cargo_env = [ 'CARGO_HOME=' + build_root / 'cargo-home' ]
 
 cargo_release = custom_target(
   'cargo-build',