Makefiles use the following conventions

I. Variables
Because makefiles variables can be specified in two different ways: ${...} and
$(...) and the variables name written with upper or lower case the following
convention will be used:

a) program names and its options will always appear with ${...} and written in 
   upper case.
b) global variables will always appear with $(...) and written in upper case.
c) local variables to a makefile will always appear with $(...) and written in
   lower case.