cabal: add no-msse4.2 flag

- Closes haskell-unordered-containers#464

unordered-containers.cabal

@@ -42,6 +42,13 @@ flag debug
   description:  Enable debug support
   default:      False
 
+flag no-msse4.2
+  description: Instruct GHC to *not* use -msse4.2. Typical use is for older
+               architectures to use unordered-containers. Expect performance
+               regressions on newer architectures.
+  default:      False
+
+
 library
   exposed-modules:
     Data.HashMap.Internal
@@ -69,7 +76,15 @@ library
     MagicHash,
     BangPatterns
 
-  ghc-options: -Wall -O2 -fwarn-tabs -ferror-spans
+  if flag(no-msse4.2)
+    ghc-options: -Wall -O2 -fwarn-tabs -ferror-spans
+  else
+    -- only enable flag on x86
+    if arch(i386) || arch(x86_64)
+      ghc-options: -Wall -O2 -fwarn-tabs -ferror-spans -msse4.2
+    else
+    -- else, get whatever GHC gives you
+    ghc-options: -Wall -O2 -fwarn-tabs -ferror-spans
 
   -- For dumping the generated code:
   -- ghc-options: -ddump-simpl -ddump-stg-final -ddump-cmm -ddump-asm -ddump-to-file