Hola,
he tenido problemas al hacer make, lo he solucionado agregando la libreria #include <errno.h> en el fichero variable.c , dejo log por si puede ayudar a alguien más:

vmod-variable-1.0$ make
make all-recursive
make[1]: Entering directory '/usr/src/varnish-vmods/vmod-variable-1.0'
Making all in src
make[2]: Entering directory '/usr/src/varnish-vmods/vmod-variable-1.0/src'
python /usr/src/varnish/varnish-4.1.2/lib/libvcc/vmodtool.py --strict ../src/variable.vcc
/bin/sh ../libtool --tag=CC --mode=compile gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I.. -I/usr/src/varnish/varnish-4.1.2/include -I/usr/src/varnish/varnish-4.1.2/bin/varnishd -I/usr/src/varnish/varnish-4.1.2 -DLOCALSTATEDIR=\"/usr/local/var\" -g -O2 -MT variable.lo -MD -MP -MF .deps/variable.Tpo -c -o variable.lo variable.c
libtool: compile: gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I.. -I/usr/src/varnish/varnish-4.1.2/include -I/usr/src/varnish/varnish-4.1.2/bin/varnishd -I/usr/src/varnish/varnish-4.1.2 -DLOCALSTATEDIR=\"/usr/local/var\" -g -O2 -MT variable.lo -MD -MP -MF .deps/variable.Tpo -c variable.c -fPIC -DPIC -o .libs/variable.o
variable.c: In function 'symtab_rehash':
variable.c:150:10: error: 'E2BIG' undeclared (first use in this function)
return E2BIG;
^
variable.c:150:10: note: each undeclared identifier is reported only once for each function it appears in
variable.c: In function 'symtab_remove':
variable.c:182:11: error: 'ENOENT' undeclared (first use in this function)
return ENOENT;
^
variable.c: In function 'symtab_get_index':
variable.c:219:11: error: 'ENOENT' undeclared (first use in this function)
return ENOENT;
^
variable.c: In function 'symtab_lookup_or_install':
variable.c:262:5: error: 'errno' undeclared (first use in this function)
errno = ENOMEM;
^
variable.c:262:13: error: 'ENOMEM' undeclared (first use in this function)
errno = ENOMEM;
^
variable.c: In function 'str2duration':
variable.c:589:2: error: 'errno' undeclared (first use in this function)
errno = 0;
^
variable.c: In function 'setval':
variable.c:684:3: error: 'errno' undeclared (first use in this function)
errno = 0;
^
Makefile:470: recipe for target 'variable.lo' failed
make[2]: *** [variable.lo] Error 1
make[2]: Leaving directory '/usr/src/varnish-vmods/vmod-variable-1.0/src'
Makefile:422: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory '/usr/src/varnish-vmods/vmod-variable-1.0'
Makefile:353: recipe for target 'all' failed
make: *** [all] Error 2

Muy buen proyecto, necesitaba este tipo de variables por session para peticion http. Muchas gracias.