C++ question - linker failed

I defined a constant boolean in an external header file:
const bool CONDITION = false ;

Then I wrote in a function the following line:
void func(...) {
...
if (cond) {
   if (CONDITION) {
      ...
   }
   else
      ...
...
}

Then g++ couldn't compile the code, reporting that the linker couldn't find the function.

I haven't figured out why it happended, but a solution is changing the if statement and constant bool value into a #ifdef directive and a macro.