Add -Wunused-const-variable=2 on GCC >=6.1

This caused a trivial warning in curve25519-donna-64bit.h, which
had two unused constants.  I commented them out.

configure.ac

@@ -1778,6 +1778,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [
      CFLAGS="$CFLAGS -Wignored-attributes -Wshift-negative-value -Wshift-overflow=2"
      CFLAGS="$CFLAGS -Wnull-dereference"
      CFLAGS="$CFLAGS -Wduplicated-cond"
+     CFLAGS="$CFLAGS -Wunused-const-variable=2"
   fi
 
   if test "x$have_shorten64_flag" = "xyes"; then

src/ext/ed25519/donna/curve25519-donna-64bit.h

@@ -8,9 +8,9 @@
 
 typedef uint64_t bignum25519[5];
 
-static const uint64_t reduce_mask_40 = ((uint64_t)1 << 40) - 1;
+//static const uint64_t reduce_mask_40 = ((uint64_t)1 << 40) - 1;
 static const uint64_t reduce_mask_51 = ((uint64_t)1 << 51) - 1;
-static const uint64_t reduce_mask_56 = ((uint64_t)1 << 56) - 1;
+//static const uint64_t reduce_mask_56 = ((uint64_t)1 << 56) - 1;
 
 /* out = in */
 DONNA_INLINE static void