Test target feature computation

Author: ranma42

src/test/run-make/print-cfg/Makefile

@@ -5,6 +5,7 @@ all: default
 	$(RUSTC) --target x86_64-pc-windows-gnu --print cfg | grep x86_64
 	$(RUSTC) --target i686-pc-windows-msvc --print cfg | grep msvc
 	$(RUSTC) --target i686-apple-darwin --print cfg | grep macos
+	$(RUSTC) --target i686-unknown-linux-gnu --print cfg | grep sse2
 
 ifdef IS_WINDOWS
 default: