Also run generate.py if vmod_abi.h or vcs_version.h are missing
Also run generate.py if vmod_abi.h or vcs_version.h are missing

Fixes #2597

Merges #2600 slightly modified

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache