Check that 'style' is compiled in a supported configuration.

Provide an explanatory error message when the compilation would fail because of missing or duplicate items.

components/style/build.rs

@@ -94,6 +94,15 @@ fn generate_properties() {
 }
 
 fn main() {
+    let gecko = cfg!(feature = "gecko");
+    let servo = cfg!(feature = "servo");
+    if !(gecko || servo) {
+        panic!("The style crate requires enabling one of its 'servo' or 'gecko' feature flags");
+    }
+    if gecko && servo {
+        panic!("The style crate does not support enabling both its 'servo' or 'gecko' \
+                feature flags at the same time.");
+    }
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:out_dir={}", env::var("OUT_DIR").unwrap());
     generate_properties();