crc32.c is largely equivalent to the freebsd implementation

Revert "use includes from VARNISHSRC if we need to" -
This reverts commit 496725d6.

The decision to pull in crc32 is based on an irc dicussion with phk
that vmods should not use libvgz. The libvgz api is not to be exposed
to minimize name clash issues with libz. As we only need crc32, there
is no good reason to use libvgz in the first place.

if it is an alternative one and we got a preferred backend