doc: add ctime_tests usage instructions to README #1803

1. 
   8144225309 commented at 3:20 pm on January 22, 2026: none

   Motivation

   When building with CMake and running ctime_tests, users see a message suggesting libtool, which is Autotools-specific:

   Before

   0$ ./build/bin/ctime_tests
   1This test can only usefully be run inside valgrind because it was not compiled under msan.
   2Usage: libtool --mode=execute valgrind ./ctime_tests
   

   This is confusing for CMake users since they don’t have libtool. Reported in #1697.

   Solution

   Following the approach in #1778, replace the build-system-specific command with a pointer to README.md and document both methods there.

   After

   0$ ./build/bin/ctime_tests
   1This test can only usefully be run inside valgrind because it was not compiled under msan.
   2See README.md for usage instructions.
   

   The README now includes instructions for both build systems.

   Fixes #1697

2. doc: add ctime_tests usage instructions to README

   The usage hint in ctime_tests.c suggests libtool, which only
   applies to Autotools builds. Point to README.md instead.
   
   Fixes #1697

   a78c1450b5
3. 
   hebasto commented at 3:23 pm on January 22, 2026: member

   Concept ACK.
4. 8144225309 closed this on Jan 22, 2026

   ---

5. 
   8144225309 commented at 3:42 pm on January 22, 2026: none

   do you like this style better or #1804 ??
6. 
   real-or-random commented at 4:17 pm on January 22, 2026: contributor

   I prefer #1804.
7. real-or-random added the label tweak/refactor on Jan 22, 2026
8. real-or-random added the label meta/development on Jan 22, 2026
9. real-or-random removed the label meta/development on Jan 22, 2026
10. real-or-random added the label user-documentation on Jan 22, 2026

Contributors
8144225309 hebasto real-or-random

Labels
user-documentation tweak/refactor