Newer versions of the compilers have apparently set the source level
switches differently, so that explicit_bzero() is not defined as our
source is currently written. The autoconf AC_CHECK_FUNCS check
evidently sets certain source switches, but is not transparent
about which ones (this has become evident with strdup() as well).

On my machine, right now, defining _DEFAULT_SOURCE appears to do
the trick. But it is not obvious what else that turns on and off.

It is poor practice to cargo-cultishly turn on GNU_SOURCE for
everything. That risks portability and compatibility, and may make
us unwittingly dependent on non-standard features that may or may
not be supported elsewhere.

Besides, we only need ZERO_OBJ() for FREE_OBJ(), it frankly doesn't
matter much if we zero an object whose pointer is about to be free'd.

... after the changes in a80c4f14d2ec046a307a4a041d7647377a7a9a4c

At least for now, we need the vcc private interface because of the
nature of the problem we are solving with this vmod.

This allows for builds against Varnish trunk or any numbered
version; but it doesn't check for a minimum version. In branch 6.0,
we have VARNISH_PREREQ([6.0.0]), which can check against numbered
versions, but fails for Varnish trunk.

ref: 0c74d1b99f1f43fc99725fe14322760afa7858ee

Makes it possible to see the varnishtest command invoked by check,
so as to use it to test individual vtc's.

These are formatted as for VCC_PrintCName() in libvcc.

Only free elements of the VCL table if it is not NULL, and then
don't forget to free the array.