test: show both CMake and Autotools usage for ctime_tests #1804

When building with CMake and running ctime_tests outside valgrind, users see:

Usage: libtool --mode=execute valgrind ./ctime_tests

CMake users don't have libtool. Show both commands.

Before

$ ./build/bin/ctime_tests
This test can only usefully be run inside valgrind because it was not compiled under msan.
Usage: libtool --mode=execute valgrind ./ctime_tests

After

$ ./build/bin/ctime_tests
This test can only usefully be run inside valgrind because it was not compiled under msan.
Usage: valgrind ./ctime_tests (or with Autotools: libtool --mode=execute valgrind ./ctime_tests)

Fixes #1697

utACK 1bc74a22f87dd7c3da706095a06765db1bbfc7c1