Includes a workaround for the problem in Varnish issue #2508.

And fix duplicate targets in src/Makefile.

Ensure that the retrieved object was saved via set.add().

Checked with gcc/g++ and clang/clang++.

Otherwise builds can fail due to partially written files from vmodtool
running on another target.

Take advantage of the fact that the array of indexes returned from
the vre2 call is now sorted (but still using a linear search).

for some early versions of re2.

Also get rid of an unnecessary #include "config.h".

Reduce code duplication for set.string() and .backend().

vcl.h must come before vrt.h for the VMOD event function typedef.

See Varnish issue #2449