bsdmainutils fails to build from source on musl-linux-any, because the
musl C library does not provide a sys/cdefs.h header. This header is
being included for using __FBSDID. However. the relevant Makefile also
adds "-include bsd/string.h" to the compiler flags and that happens to
provide a definition of __FBSDID. Therefore, we can delete sys/cdefs.h
at no loss of functionality and thus make the build pass. I'm attaching
a patch for your convenience.

Helmut