VPATH
Do not set VPATH to the value of another variable, for example
‘VPATH = $(srcdir)’, because some ancient versions of
make do not do variable substitutions on the value of
VPATH. For example, use this
srcdir = @srcdir@
VPATH = @srcdir@
rather than ‘VPATH = $(srcdir)’. Note that with GNU Automake, there is no need to set this yourself.