Doe not compile on Ubuntu xenial against varnish-5.1.3 source #2

When I run:
./configure VARNISHSRC=/root/varnish-5.1.3 VMODDIR=/usr/lib/varnish/vmods

It fails with error:

checking for /root/varnish-5.1.3/include/varnishapi.h... no
configure: error: in `/root/libvmod-ldap':
configure: error: "/root/varnish-5.1.3" is not a Varnish source directory

although I've the source fetched via apt-get source varnish=5.1.3. Same result when I tried with the downloaded source tarball. Also tried to build the varnish source first but that didn't make any difference either.

Am I doing something wrong or the module is not compatible with varnish-5.1?