GNU make - Overriding Go to the first , previous , next , last section, table of contents .
The most common way to use this facility is to pass extra flags to compilers. Each time you run make , you can override this value if you wish. See section Variables Used by Implicit Rules , for a complete list. You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables. But, unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.