Don't use SSE4.1 instructions unless the target supports it

Fixes #48.

Enabling the SIMD code path now requires building with
`RUSTFLAGS="-C target-feature=+sse4.1"` or similar.

Author: mbrubeck

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "euclid"
-version = "0.16.2"
+version = "0.16.3"
 authors = ["The Servo Project Developers"]
 description = "Geometry primitives"
 documentation = "https://docs.rs/euclid/"

src/lib.rs

@@ -7,7 +7,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![cfg_attr(feature = "unstable", feature(asm, repr_simd, test, fn_must_use))]
+#![cfg_attr(feature = "unstable", feature(asm, cfg_target_feature, repr_simd, test, fn_must_use))]
 
 //! A collection of strongly typed math tools for computer graphics with an inclination
 //! towards 2d graphics and layout.

src/side_offsets.rs

@@ -202,13 +202,13 @@ impl SideOffsets2DSimdI32 {
         }
     }
 
-    #[cfg(not(target_arch = "x86_64"))]
+    #[cfg(not(target_feature = "sse4.1"))]
     #[inline]
     pub fn is_zero(&self) -> bool {
         self.top == 0 && self.right == 0 && self.bottom == 0 && self.left == 0
     }
 
-    #[cfg(target_arch = "x86_64")]
+    #[cfg(target_feature = "sse4.1")]
     #[inline]
     pub fn is_zero(&self) -> bool {
         let is_zero: bool;