Both GCC and Clang ship their own stddef.h which does this (musl's
stddef.h is simply ignored). But, musl also defines the macro in a
number of other headers. Thus, depending on which header you include
last, you'll end up with a different definition of NULL.

Mostly, getting musl's definition simply degrades warning diagnostics
in C++ slightly -- e.g. GCC can no longer emit this warning:
  warning: passing NULL to non-pointer argument 1 of 'int foo(long int)'
[-Wconversion-null]

If you're using Clang's modules support, it can also break
compilation. In that case, the conflicting definitions may be detected
as a module incompatibility.

A different (potentially better) fix would be to always retrieve the
definition of NULL from the compiler's stddef.h (via #define
__need_NULL #include <stddef.h>). It may also be best to delete the
musl stddef.h entirely for clarity, since it's currently ignored,
anyhow.

But, this seemed the more minimal fix.