vmodtool runs git in the directory of the input file. For an out-of-tree
build, this resides in the build dir, which has no git.

with the introduction of hdr_t, headers are no longer just pascal
strings, they are a struct consisting of an unsigned char length and a
char array for name and value. But the binary representation is
unchanged, so I really see no point in inventing complications and just
cast back and forth.

Since VC 7.7, vmodtool.py generates a file that
must be taken into account in Makefile.am
https://varnish-cache.org/docs/trunk/whats-new/upgrading-7.7.html

Not sure if we should backport the .get() fallback change, but if we
want to, this commit should do it.

Because we can not provide defaults for all types, .get() might trigger
VCL failure now.

Note that the interface is fully compatible with existing VCL, but while
previously a missing fallback argument would trigger a compile time
error, it will result in a VCL failure now.

The types without a default which might trigger a failure are:

* ACL: Trying to match a NULL ACL would trigger a failure in
  varnish-cache anyway

* BODY: Due to the lack of examples, it is unclear what a NULL body
  would look like

  A sensible default might be added later (maybe the empty body?)

* HEADER: A default makes no sense

- Include the Varnish default vmod_path in the varnishtest command.
- Non-generated VTCs are also distrubuted.
- Clean the .dummy file.

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

we use a single malloc for struct vrt_blob and the blob data it points
to

- support out-of-tree builds
- explicitly define required sources
- differentiate built sources
- avoid temporary files in srcdir