overwrite HOST_* flags to get rid of -MF

--- third_party/rust/glslopt/build.rs	2020-06-03 01:37:31.000000000 +0200
+++ third_party/rust/glslopt/build.rs	2020-06-04 10:57:16.418469546 +0200
@@ -28,6 +28,8 @@
     env::remove_var(format!("CXXFLAGS_{}", &target));
     env::remove_var(format!("CFLAGS_{}", target.replace("-", "_")));
     env::remove_var(format!("CXXFLAGS_{}", target.replace("-", "_")));
+    env::set_var("HOST_CFLAGS", "-O2");
+    env::set_var("HOST_CXXFLAGS", "-O2");
 
     configure(&mut cc::Build::new())
         .warnings(false)