Use this mini cheat-sheet while adopting the rules:

VAR = value Assign value to VAR
VAR += value Append value to VAR list
VAR -= value Remove value from VAR list
$$VAR or $${VAR} Gets VAR’s value at the time qmake is running
$(VAR) Contents of an Environment VAR at the time Makefile (not qmake) is running
$$(VAR) Contents of an Environment VAR at the time qmake (not Makefile) is running