How to construct CMake files to be OS generic but specific too?

Actually, my project has this files:

/ cmake / CMakeLists.txt
/ sources / {my cpp headers ...}

And I construct the "project" (makefile or .sln...) the same way under every OS, at the root of this repository with the command:

cmake ./cmake/

But it needs some optional settings like the CMAKE_CXX_FLAGS different on each OS.

And I have chosen to put this variables in a /cmake/CMakeCache.txt but to not commit this file (since it is different between OSs). The dev has to generate (edit) this file on each machine. I only gave some instructions in a readme about this CMakeCache file.

How could I make cmake files generic again, but have this differences in a commited content too?

Solution

Use code like this in your CMakeLists.txt:

if(UNIX AND NOT APPLE)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -pthread")
elseif(WIN32)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DWIN32_LEAN_AND_MEAN")
endif()

Notes:

Append your options to ${CMAKE_CXX_FLAGS} so that user-specified settings are also included.
You cannot use list(APPEND ...) with compiler flags because it uses semicolons to separate parameters (this is why the code above uses set(...) with quotes)
APPLE is also UNIX, so we need NOT APPLE when we actually mean Linux