cargo: don't build when it's not set

sys/Cargo.toml

@@ -19,9 +19,9 @@ default  = ["avcodec", "avdevice", "avfilter", "avformat", "avresample", "swresa
 static = []
 
 build          = ["static"]
-build-gpl      = []
-build-nonfree  = []
-build-version3 = []
+build-gpl      = ["build"]
+build-nonfree  = ["build"]
+build-version3 = ["build"]
 
 avcodec    = []
 avdevice   = ["avformat"]

sys/build.rs

@@ -231,6 +231,10 @@ fn build() -> io::Result<()> {
 }
 
 fn main() {
+	if env::var("CARGO_FEATURE_BUILD").is_err() {
+		return;
+	}
+
 	println!("cargo:rustc-link-search=native={}", search().join("lib").to_string_lossy());
 
 	if fs::metadata(&search().join("lib").join("libavutil.a")).is_ok() {