$(HOSTNAME) not set in a makefile?

["H.S." <hs.samix@gmail.com>]

In a makefile, if I have a rule to:
	echo $(HOSTNAME);

it return a blank line, as if the variable is not set. Echoing $(HOME) 
works, $(LANG) works, what is the deal with $(HOSTNAME)?

Echoing $(HOSTNAME) on a command prompt works though.

->HS