sbbs fails to compile on ubuntu 21.10

bbs@testbbs:~/repo/3rdp$ gcc -v 2>&1|tail -1 gcc version 11.2.0 (Ubuntu 11.2.0-7ubuntu2)

At this stage Linking gcc.linux.x64.exe.release/sbbs g++ -Lgcc.linux.x64.lib.release -lutil -L/home/bbs/repo/src/sbbs3/../uifc/gcc.linux.x64.lib.release -L/home/bbs/repo/src/sbbs3/../xpdev/gcc.linux.x64.lib.release -L/home/bbs/repo/src/sbbs3/../smblib/gcc.linux.x64.lib.release -L/home/bbs/repo/src/sbbs3/../encode/gcc.linux.x64.lib.release -L/home/bbs/repo/src/sbbs3/../hash/gcc.linux.x64.lib.release -L/home/bbs/repo/src/sbbs3/../conio/gcc.linux.x64.lib.release -L/usr/local/lib -L/usr/lib/x86_64-linux-gnu -L/home/bbs/repo/src/sbbs3/../../3rdp/gcc.linux.x64.release/mozjs/lib -L/home/bbs/repo/src/sbbs3/../../3rdp/gcc.linux.x64.release/cl -o gcc.linux.x64.exe.release/sbbs gcc.linux.x64.obj.release-mt/sbbscon.o gcc.linux.x64.obj.release-mt/sbbs_status.o gcc.linux.x64.obj.release-mt/sbbs_ini.o -lcap -lsbbs -lftpsrvr -lwebsrvr -lmailsrvr -lservices -lsmb -lxpdev_mt -lm -lpthread -ldl

/usr/bin/ld: gcc.linux.x64.lib.release/libsbbs.so: undefined reference to `pthread_yield' collect2: error: ld returned 1 exit status

It appears pthread-yield is no longer in libpthread.so in Ubuntu 21.10

bbs@testbbs:/lib/x86_64-linux-gnu$ nm -s libpthread.so.0|grep yield bbs@testbbs:/lib/x86_64-linux-gnu$

Whereas on 20.04: root@wibble:~# nm -s /usr/lib/x86_64-linux-gnu/libpthread.so|grep yield 000000000000ad70 T pthread_yield U sched_yield@@GLIBC_2.2.5

The compilation issue was first noticed by rjwboys on #sycnrhonet and verified by me.