New autoconf macros for VMOD maintainers

[Dridi Boukelmoune]

Hello everyone,

I just pushed 4 commits [1] to master for inclusion in Varnish 5.0
with what I hope is proper documentation [2,3]. I immediately back-ported
the commits to the 4.1 branch, here is why:

New macros are documented as "since 4.1.4", but future hypothetical
changes to the macros will not be back-ported to 4.1 to ensure a clean
upgrade path. For instance, something that would be in 4.1.5 and 5.0.1
but not in 5.0.0. Not getting it in the next 4.1 would be a loss IMO since
it is still supported.

Regarding the macros themselves, they only introduce new facilities
for out-of-tree VMOD maintainers, and although it's been tested on
many platforms, I hope maintainers will let us know if they break in
unexpected ways. Expected failures are basically old versions of
autotools installations.

The "us" in "let us know" can safely be turned into me. From now on
I volunteer for autotools maintenance. This libvmod-painkiller journey
taught me a lot about this stuff, and also I don't quite like it, I can now
appreciate it better.

Once 5.0 is branched, I'll start working on cleaning master up so that
only Varnish developers/contributors should be affected in which case
they can throw their crap at me (subtle joke hidden inside that last
sentence).

If anyone has objections, let me know. It's not too late to revert the
commits too.

Best,
Dridi

[1] https://github.com/varnishcache/varnish-cache/compare/edd4ebe...cd29eda
[2] https://github.com/varnishcache/varnish-cache/blob/cd29eda/varnish.m4#L160-L237
[3] https://github.com/varnishcache/varnish-cache/blob/cd29eda/varnish.m4#L244-L253