How to define a C++ preprocessor macro through the command line with CMake?

A good alternative would be to define a cmake option:

OPTION(DEFINE_MACRO "Option description" ON) # Enabled by default

Followed by a condition:

IF(DEFINE_MACRO)
    ADD_DEFINITIONS(-DMACRO)
ENDIF(DEFINE_MACRO)

Then you can turn that option ON/OFF via command line with cmake using the -D flag. Example:

cmake -DDEFINE_MACRO=OFF ..

To make sure the compiler is receiving the definition right, you can call make in verbose mode and check for the macro being defined or not:

make VERBOSE=1

This is a good solution also because make will recompile your code when any of cmake options changes.

Try this: -D CMAKE_CXX_FLAGS=/DMY_MACRO=1

The motivation behind the question was to batch build 3rd party libraries, which is why I wanted to avoid modifying CMakeLists. So years later, even though I don't need that anymore, I figured out that it's easily achievable by means external to CMake:

Invoke CMake as usual, no special flags.
Then:
- With MSVC: The compiler reads the CL environment variable to get extra command line arguments. So
```
  set CL=/DMY_MACRO=1 %CL%
```
  then invoke MSBuild to do its job.
- With Makefiles: The generated makefiles use the CFLAGS and CXX_FLAGS variables as makefiles are expected to do. So the build can be started by
```
  make CXX_FLAGS=-DMY_MACRO=1
```
  or by setting the corresponding environment variables.