The PATH_MAX symbol is often, but not always, defined
in the <limits.h> header.  In particular, on cygwin you
need to include this header to avoid a compilation error.

However, some systems define PATH_MAX to be something as
small as 256, which POSIX is happy to allow, while others
allow much larger values.  In general it can vary from
one filesystem to another.

In order to avoid the vagaries of different systems, define
our own symbol.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>