This shows up only when there is no sys/cdefs.h on a system at all, such as when building on a musl-libc host.

sys/cdefs.h is not present at all on musl libc systems, so perhaps this should point at bsd/sys/cdefs.h instead.

Example failed build output:

```
clang -std=c11 -I../include/ -O -g -Wall -Wextra -Wpedantic -Wno-missing-field-initializers -Werror -O3 -fstack-protector-strong -D_FORTIFY_SOURCE=2 -finstrument-functions   -c -o bworker.o bworker.c
In file included from bworker.c:10:
In file included from /usr/include/bsd/stdlib.h:39:
/usr/include/bsd/libutil.h:43:10: error: 'sys/cdefs.h' file not found with <angled> include; use "quotes" instead
#include <sys/cdefs.h>
         ^
In file included from bworker.c:10:
/usr/include/bsd/stdlib.h:45:10: error: 'sys/cdefs.h' file not found with <angled> include; use "quotes" instead
#include <sys/cdefs.h>
         ^
In file included from bworker.c:11:
/usr/include/bsd/sys/tree.h:33:10: fatal error: 'sys/cdefs.h' file not found
#include <sys/cdefs.h>
         ^~~~~~~~~~~~~
3 errors generated.
make: *** [Makefile:16: bworker.o] Error 1
```