Fixes #5

The Varnish version appears only once, making it hard to break on
Varnish upgrades.

Although it may build with 4.1.2+, stick to the branching scheme.

Libtool itself requires most of the compilation toolchain, so we don't
need to duplicate effort.

Make it choke on warnings, set up the test driver and use silent rules.

It is now replaced by AC_CANONICAL_TARGET but regardless, the $target
variable isn't used anywhere.

Require a more recent autoconf, and name the dist archive vmod-example
to match the usual package naming scheme. Update the copyright notice.

This partly reverts c7087ebc

I am aware that creating dependencies on vcc_if.h doesn't look as nice
as having a vcc target, but parallel builds just don't work otherwise

@fgsch is right, the additional path elements should not be needed

use the python we found in case it's not called "python"

Varnish is massively multithreaded, so all vmods must be thread-safe.

While it is possible to write thread-safe vmods without pthreads support,
some systems (like those of the Solaris-heritage) need additional compiler
flags for MT support (-D_REENTRANT -pthreads). To avoid weird potential
issues, we should, by default, include full MT support.