Set i686 as the minimum architecture on 32-bit mingw

Signed-off-by: Austin Seipp <austin@well-typed.com>

aclocal.m4

@@ -526,6 +526,9 @@ AC_DEFUN([FPTOOLS_SET_C_LD_FLAGS],
     esac
 
     case $$1 in
+    i386-unknown-mingw32)
+        $2="$$2 -march=i686"
+        ;;
     i386-apple-darwin)
         $2="$$2 -m32"
         $3="$$3 -m32"