test: show both Autotools and CMake usage for ctime_tests

The existing message only shows the libtool command, which is
specific to Autotools builds.

Fixes #1697

src/ctime_tests.c

@@ -49,7 +49,7 @@ int main(void) {
 
     if (!SECP256K1_CHECKMEM_RUNNING()) {
         fprintf(stderr, "This test can only usefully be run inside valgrind because it was not compiled under msan.\n");
-        fprintf(stderr, "Usage: libtool --mode=execute valgrind ./ctime_tests\n");
+        fprintf(stderr, "Usage: valgrind ./ctime_tests (or with Autotools: libtool --mode=execute valgrind ./ctime_tests)\n");
         return EXIT_FAILURE;
     }
     ctx = secp256k1_context_create(SECP256K1_CONTEXT_DECLASSIFY);