Be explicit that we want not only nacl but nacl-with-a-fast-curve25519

Resolves the user experience part of #8014.

changes/bug8014

@@ -0,0 +1,5 @@
+  o Minor usability improvements (build):
+    - Clarify that when autconf is checking for nacl, it is checking
+      specifically for nacl with a fast curve25519 implementation.
+      Fixes bug 8014.
+

configure.ac

@@ -693,7 +693,7 @@ if test x$enable_curve25519 != xno; then
   AC_CHECK_HEADERS([crypto_scalarmult_curve25519.h \
                     nacl/crypto_scalarmult_curve25519.h])
 
-  AC_CACHE_CHECK([whether we can use curve25519 from nacl],
+  AC_CACHE_CHECK([for nacl compiled with a fast curve25519 implementation],
     tor_cv_can_use_curve25519_nacl,
     [tor_saved_LIBS="$LIBS"
      LIBS="$LIBS -lnacl"
@@ -705,7 +705,7 @@ if test x$enable_curve25519 != xno; then
 	 #include <nacl/crypto_scalarmult_curve25519.h>
 	 #endif
          #ifdef crypto_scalarmult_curve25519_ref_BYTES
-	 #error Hey, this is the reference implementation!
+	 #error Hey, this is the reference implementation! That's not fast.
 	 #endif
        ], [
 	 unsigned char *a, *b, *c; crypto_scalarmult_curve25519(a,b,c);