make and curly braces

[Lois Gerber]

 Does anyone know the historical reasons behind the following
 reference on the make(1) man page:

	The -n option specifies printing of a command line without
	execution;  however, if the command line has the string
	$(MAKE) in it . . . . . . . the line is always executed.
	. . . . . Note that this feature does not work of MAKE
	is enclosed in braces, as in ${MAKE}. 

 What was the past significance of curly braces in makefiles?

 Thanks for whatever information anyone has.

 Lois Gerber
 Hewlett Packard, Fort Collins