[Issue 20727] New: `make -f posix.mak clean` requires a D compiler

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20727

          Issue ID: 20727
           Summary: `make -f posix.mak clean` requires a D compiler
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

Currently, running `make -f posix.mak clean` errors out if one doesn't have
`dmd` in the path, because the check in in `src/posix.mak` is done regardless
of the target.

However, the clean target doesn't depend on DMD.
See
https://github.com/dlang/dmd/blob/05388e29001a83d1ef47f951433528ea8cf3332c/src/posix.mak#L79-L100

This also broke `setup.sh` from the tools repository, which was one of the pain
point of Andrei: https://forum.dlang.org/post/r5e8uj$i1b$1@digitalmars.com

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