Require recent autoconf.

On EL6 systems the packaged 2.63 version is missing AS_VAR_COPY.
This seems to have been added in 2.63b, released sometime in 2008.

With this commit we cut off any vmod development being done on
EL6 and earlier. (building and running should work)

Conflicts:
	configure.ac
So much for wildcards.

Conflicts:
	Makefile.am
Add /usr/sbin to path when looking for varnishd.
Pass VMOD_DIR to make distcheck

The VMOD_DIR returned from pkg-config does not take prefix and similar
variables into account. Make sure to pass an explicit VMOD_DIR that
the build process has write access to.

Fixes #8

Conflicts:
	Makefile.am
Require libvarnishapi.

Exit with an error if we can't find libvarnishapi, instead of
just warning about missing macros.

Conflicts:
	configure.ac