How should I count the lines of code in a project?

In C++ for example, should I count the lines of the header files with my class definition? Should I count the header of a (non standard) library I am linking against?

In Qt (or any other GUI framework), should I count the lines of the form's design?

Should I count the Makefile too?

I ask because it is a simple way to measure the size of the project and/or effort involved. Even though a GUI file is not code per se the programmer might have spent hours doing it.