Factor out special casing of MSVC crt

This avoids the possibility of a duplicate or conflicting crt-static
command line option sent to rustc.

Author: smaeul

src/bootstrap/bin/rustc.rs

@@ -207,11 +207,6 @@ fn main() {
             }
         }
 
-        if target.contains("pc-windows-msvc") {
-            cmd.arg("-Z").arg("unstable-options");
-            cmd.arg("-C").arg("target-feature=+crt-static");
-        }
-
         if let Ok(s) = env::var("RUST_CRT_STATIC") {
             if s == "true" {
                 cmd.arg("-Z").arg("unstable-options");

src/bootstrap/lib.rs

@@ -886,8 +886,12 @@ impl Build {
 
     /// Returns if this target should statically link the C runtime, if specified
     fn crt_static(&self, target: &str) -> Option<bool> {
-        self.config.target_config.get(target)
-            .and_then(|t| t.crt_static)
+        if target.contains("pc-windows-msvc") {
+            Some(true)
+        } else {
+            self.config.target_config.get(target)
+                .and_then(|t| t.crt_static)
+        }
     }
 
     /// Returns the "musl root" for this `target`, if defined