VPATH
From MDC
VPATH is a standard variable that tells make where to look for files. See the GNU Make documentation on VPATH for more details.
[edit] Set By
Tree makefiles set VPATH in the standard makefile header before including autoconf.mk.
[edit] Example
In the Mozilla build system, VPATH is always set to @srcdir@, which is an autoconf variable which points to the directory to find source files.
VPATH = @srcdir@