Commit 631923e7 used a few macros from sys/wait.h without including
it. On Linux, they were also defined in stdlib.h, but on FreeBSD
the build failed:

../../tests/commandtest.c: In function 'test1':
warning: implicit declaration of function 'WIFEXITED'
warning: nested extern declaration of 'WIFEXITED' [-Wnested-externs]