Felix S. Klock II 62aa899e3d Make build timestamp files robust in face of concurrent source modification.
Strategy: If the end goal is to touch e.g. `stamp.std`, then we first
touch `stamp.std.start_time` before doing anything else.  Then when
the receipe finishes, we touch `stamp.std` using the timestamp from
`stamp.std.start_time` as the reference time, and remove
`stamp.std.start_time`.

Fix #6518.
2015-03-03 15:11:01 +01:00
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