How do I debug CMakeLists.txt files?

I like to use variable_watch to "debug" my CMakeLists.txt files. Just set in top of my script:

variable_watch(SOME_MY_VAR)

You can try using the new CMake Script Debugger provided by the VisualGDB tool. It uses an open-source fork of CMake that supports stepping through CMakeLists.txt files, setting code/data breakpoints, evaluating/changing variables, etc.

There's a detailed step-by-step tutorial on the new debugger here

There is no interactive debugger for CMake, however there are also the flags -Wdev, --debug-output and --trace which might help. Also remember to check the log files CMakeFiles\CMakeOutput.log and CMakeFiles\CMakeError.log which mainly collect outputs of processes called by CMake (for example while checking for presence of a type or header).

Since version 3.7, CMake now officially supports a "server mode" so integration in IDEs is likely to improve in the near future. Initial support exists both in Qt Creator and Visual Studio 2017 RC

There are steveire's CMake Daemon Tools. I haven't used them myself, but they claim to offer possibilities for introspection that seem to be pretty close to a debugger.

Edit: They are now called CMake-server and are part of CMake 3.7.