The VARNISH_TEST_PATH variable contains the minimal PATH for running a
test suite, covering only Varnish tools. And VARNISH_LIBRARY_PATH is
needed because Varnish binaries aren't build using an RPATH and only
know about their libraries out of the box when they are installed in
the system path, via ldconfig.

Consequently VMOD_TEST_PATH is now an alias for VARNISH_TEST_PATH.

Catching this problem after the 5.0 release is unfortunate, as it won't
be ported to 4.1 to avoid breaking the upgrade path between the 4.1
series and the 5.0.0 release. Unless it is considered OK?

Do people really read through the whole commit message and diff when it
concerns autotools changes?

While at it, make a pass on the autoconf documentation. Flatten code
snippets that don't really need line breaks. Add the man page to the
dist tarball by default. Walk through a typical test suite setup.
Document variables created or substituted by AC_PREREQ. Document how
the purpose and requirements of varnish.m4, and document what changes
between versions of Varnish.

Consequently bump the serial number.