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
