title: How to define a macro in a header file from the makefile
body: <p>I have written my own header file for checking assertions myassert.h, similar to the assert.h header and I include this in another file, say project_file.h.  I also have the NDEBUG macro which when defined before the inclusion of myassert.h, disables the checking of the assertions.
  However, I dont want to turn off the assertion checking by defining NDEBUG at the beginning of project_file.h. Instead I want to have a flag in the makefile so that during compilation, the user can set the flag that will automatically disable/enable the assertions checking feature. 
  The file project_file.h is just one file in a huge project. Can anyone please tell me how to do this? I have checked the other answers but I did not quite understand how to make it work.</p>
  
  <p>Thank you very much.</p>