- support out-of-tree builds
- explicitly define required sources
- differentiate built sources
- avoid temporary files in srcdir

Ref: https://github.com/varnishcache/varnish-cache/pull/3123

... now that the name clash is resolved

Ref: https://github.com/varnishcache/varnish-cache/commit/5450550906d0809dfc21ae9d3a5b14446c1809d4

This reverts commit 532fdef2.

Ref:	d67993451bda7eeb7c0ea3c5528a8353f283a190

ref: 0c74d1b99f1f43fc99725fe14322760afa7858ee

Ref: e8985f22711fe9a342b27a10052ae3655f64435c