Redesign spirv-builder's watch API

Author: tuguzT

crates/spirv-builder/src/lib.rs

@@ -125,14 +125,15 @@ pub enum SpirvBuilderError {
     BuildFailed,
     #[error("multi-module build cannot be used with print_metadata = MetadataPrintout::Full")]
     MultiModuleWithPrintMetadata,
-    #[error("watching within build scripts will prevent build completion")]
-    WatchWithPrintMetadata,
     #[error("multi-module metadata file missing")]
     MetadataFileMissing(#[from] std::io::Error),
     #[error("unable to parse multi-module metadata file")]
     MetadataFileMalformed(#[from] serde_json::Error),
     #[error("cargo metadata error")]
     CargoMetadata(#[from] cargo_metadata::Error),
+    #[cfg(feature = "watch")]
+    #[error("watching within build scripts will prevent build completion")]
+    WatchWithPrintMetadata,
 }
 
 const SPIRV_TARGET_PREFIX: &str = "spirv-unknown-";