[Issue 18120] New: Allow DRT options to be set via the environment

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18120

          Issue ID: 18120
           Summary: Allow DRT options to be set via the environment
           Product: D
           Version: D2
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: greeenify at gmail.com

>From https://github.com/ldc-developers/ldc/pull/1672#issuecomment-353788172

> For some use cases where it is not possible to add commandline flags to the executable (e.g. in scripts that one can't/shouldn't change), it'd be nice to be able to set the configuration through other means, e.g. built-in like in this PR, or probably better: through an ENV variable. ASan uses an ENV var, which is quite nice. So perhaps an idea to be able to set DRT options through an ENV variable aswell.

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