libarchive: Remove a system preprocessor macro that conflicts with a local var

On SunOS i386, the system headers sometimes define macro names
corresponding to register names, short and with no prefix.
Undefine one that conflicts with our code.

Utilities/cmlibarchive/libarchive/archive_blake2sp_ref.c

@@ -28,6 +28,9 @@
 
 #define PARALLELISM_DEGREE 8
 
+/* Remove system-defined preprocessor defintions that conflict with us.  */
+#undef FS
+
 /*
   blake2sp_init_param defaults to setting the expecting output length
   from the digest_length parameter block field.