Interpolate substitution vars in the arg (i.e. $(DEFFILE))
Example:
x[1] += 42 ^^^ (for []) x[1] += 42 ^ (for +) x[1] += 42 ^^^^^^ (for []=)
x[1] += 42 ^^^^^^^^
x.foo += 42 ^^^ (for foo) x.foo += 42 ^ (for +) x.foo += 42 ^^^^^^^ (for foo=)
x.foo += 42 ^^