https://issues.dlang.org/show_bug.cgi?id=23631

          Issue ID: 23631
           Summary: [REG master] ./src/importc.h:80:8: warning: undefining
                    "__has_builtin"
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: ibuclaw@gdcproject.org

This nonsense warning has started to appear when building druntime.

---
../compiler/../generated/linux/release/64/dmd -c -conf= -Isrc -Iimport -w -de
-preview=dip1000 -preview=fieldwise -m64 -fPIC  -preview=dtorfields -O -release
-inline  -version=Shared -fPIC -version=Shared -fPIC -I. src/core/stdc/errno.c
-of../generated/linux/release/64/errno_c.o
In file included from <command-line>:
./src/importc.h:80:8: warning: undefining "__has_builtin"
   80 | #undef __has_builtin
      |        ^~~~~~~~~~~~~

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