[Bug debug/68848] allow -fdebug-prefix-map to read OLD prefix from environment (improve reproducibility)

[dkg at fifthhorseman dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68848

Daniel Kahn Gillmor <dkg at fifthhorseman dot net> changed:

           What    |Removed                     |Added
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  Attachment #37001|0                           |1
        is obsolete|                            |

--- Comment #2 from Daniel Kahn Gillmor <dkg at fifthhorseman dot net> ---
Created attachment 37003
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37003&action=edit
allow reading OLD argument to -fdebug-prefix-map from environment (using ENV:
prefix)

the v3 patch fixes coding conventions (thanks, Bernd Schmidt!)