Each Mozilla makefile uses a standard header. This header sets variables which tell the makefile where it is and where the source directory is, and then include autoconf.mk, to pick up makefile variables which are set during configuration.

DEPTH           = ../../../..
topsrcdir       = @top_srcdir@
srcdir          = @srcdir@
VPATH           = @srcdir@

include $(DEPTH)/config/autoconf.mk