4.6. DEFCON 3 - Variable Expansion

As noted before, PMake will not expand a variable unless it knows of a value for it. This can cause problems for makefiles that expect to leave variables undefined except in special circumstances (e.g. if more flags need to be passed to the C compiler or the output from a text processor should be sent to a different printer). If the variables are enclosed in curly braces (${PRINTER}), the shell will let them pass. If they are enclosed in parentheses, however, the shell will declare a syntax error and the make will come to a grinding halt.

You have two choices: change the makefile to define the variables (their values can be overridden on the command line, since that is where they would have been set if you used Make, anyway) or always give the -V flag (this can be done with the .MAKEFLAGS target, if you want).