VPATH and Make
Posix does not specify the semantics of VPATH. Typically,
make supports VPATH, but its implementation is not
consistent.
Autoconf and Automake support makefiles whose usages of VPATH are
portable to recent-enough popular implementations of make, but
to keep the resulting makefiles portable, a package's makefile
prototypes must take the following issues into account. These issues
are complicated and are often poorly understood, and installers who use
VPATH should expect to find many bugs in this area. If you use
VPATH, the simplest way to avoid these portability bugs is to
stick with GNU make, since it is the most
commonly-used make among Autoconf users.
Here are some known issues with some VPATH
implementations.