Per the manpage in the file's comment, SIOCOUTQ is equivalent to the
TIOCOUTQ ioctl, whose value can be obtained by including <sys/ioctl.h>,
which allows us to avoid including any system-specific headers. This is
desirable so that, on a musl system, it won't be necessary to install
kernel headers in order to build epics-base.
Since it's first been checked into Git, the Linux kernel has defined
SIOCOUTQ to be TIOCOUTQ [1].
From the three main libc options on Linux: glibc and uclibc use the
kernel headers, so both ioctls are available; and musl defines only
TIOCOUTQ in their own headers.
[1] 1da177e4c3
21368dc7b4