cmake: how to check if preprocessor is defined - cmake

Question

I can't get cmake to test if a preprocessor has been defined or not. Eg:
cmake_minimum_required(VERSION 2.8.9)
project (cmake-test)
add_definitions(-DOS=LINUX)
if(NOT <what condition goes here?>)
message(FATAL_ERROR "OS is not defined")
endif()
The following tests don't work:
if (NOT COMMAND OS)
if (NOT DEFINED OS)
if (NOT OS)
I can get it to work by using set() and just testing for a regular cmake variable and then defining the preprocessor macro. Eg:
set(OS LINUX)
if (OS)
add_definitions(-DOS=${OS})
else()
message(FATAL_ERROR "OS is not defined")
endif()
In case, you're wondering why I need to test it if the variable/preprocessor is in the same file, it's because in the final implementation these will come from an external file which is includeed in the main CMakeFile.txt Eg:
include(project_defs.txt)
if (OS)
....

Answer 1

This is to complete the answer by arrowd.
I also tried the COMPILE_DEFINITIONS option as mentioned above by arrowd unsuccessfully.
Following the documentation of CMake, at least for version 3.x, it turns out that when you call add_definitions() in CMake, it adds the definitions to the COMPILE_DEFINITIONS directory property.
Therefore, lets say you are defining the following as per your code:
add_definitions(-DOS=LINUX)
To retrieve the string with the definitions added into the variable "MYDEFS" you can use the following lines in CMake:
get_directory_property(MYDEFS COMPILE_DEFINITIONS)
MESSAGE( STATUS "Compile defs contain: " ${MYDEFS} )
Then you can check if in ${MYDEFS} the define you are looking for exists or not. For instance
if(MYDEFS MATCHES "^OS=" OR MYDEFS MATCHES ";OS=")
MESSAGE( STATUS "OS defined" )
else()
# You can define your OS here if desired
endif()

Answer 2

Normally, all definitions that are passed to the compiler are controlled by CMake. That is, you create a CMake variable with
option(SOMEFEATURE "Feature description" ON)
or
set(OS "" CACHE STRING "Select your OS")
User sets them via cmake -D OS=DOS or in the CMake GUI. Then you can use if() operator to conditionally add_definitions() to the compiler command line.
UPDATE:
If you really want to access preprocessor flags, there is a COMPILE_DEFINITIONS target property. You can access it this way:
get_target_property(OUTVAR target COMPILE_DEFINITIONS)