makefile-macro-assign

makefile-macro-assign is a variable defined in `make-mode.el'.
Its value is

" = "

Documentation:
String to append to all macro names inserted by `makefile-insert-macro'.
The normal value should be " = ", since this is what
standard make expects. However, newer makes such as dmake
allow a larger variety of different macro assignments, so you
might prefer to use " += " or " := " .

You can customize this variable.