Don't panic if WASI_SDK_PATH is not set when detecting compiler

They are not always needed when building std, as is the case when
packaging on Fedora. Instead, print a warning that the default
cxx/cc compiler will be used.

Author: Paul Murphy

src/bootstrap/src/utils/cc_detect.rs

@@ -221,10 +221,12 @@ fn default_compiler(
         }
 
         t if t.contains("-wasi") => {
-            let root = build
-                .wasi_sdk_path
-                .as_ref()
-                .expect("WASI_SDK_PATH mut be configured for a -wasi target");
+            let root = if let Some(path) = build.wasi_sdk_path.as_ref() {
+                path
+            } else {
+                println!("WARNING: WASI_SDK_PATH not set, using default cc/cxx compiler");
+                return None;
+            };
             let compiler = match compiler {
                 Language::C => format!("{t}-clang"),
                 Language::CPlusPlus => format!("{t}-clang++"),