CMake add_definitions does not seem to work

The following form won't do what you expect in any case:

add_definitions(hehe_test)

According to the documentation the form is:

add_definitions(-DFOO -DBAR ...)

In your case it means:

add_definitions(-Dhehe_test)

You can also give them a value if required:

// statically defined
add_definitions(-Dfoo=bar)
// use a cmake or user-defined variable 
add_definitions(-Dbar=${FOO})
// use "" if you intend to pass strings with spaces
add_definitions(-Dxxx="${YYY}")

Note that you can put all of them together in the same add_definitions call.

In order to get the C/C++ effect of :

#define _MYSYMBOL

From the command line you can :

cmake . -DCMAKE_C_FLAGS=" -D_MYSYMBOL " -DCMAKE_CXX_FLAGS=" -D_MYSYMBOL "

From within CMakeLists.txt:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_MYSYMBOL")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_MYSYMBOL")

These add flags to the c/c++ compiler command directly. -D is define symbol

You can also use cmake's add_definitions command

add_definitions ( -D_MYSYMBOL1 -D_MYSYMBOL2 )

add_definitions makes some slight assumptions as described here