This is to resolve the chicken and egg problem of the VSC headers being
generated only once bin/varnishd is built, while they are used in several
VMODs and libraries.

This moves the generation of the VSC .c and .h header files to a directory
in lib that can be built earlier than its dependencies.

Also Don't AC_SUBST the vsc files. The complete list of vsc files is known
and listed in the .am file when automake is run, so there is no need to
have configure.ac help with making lists.