Build error, unable to link
L4T Ubuntu (Nintendo switch) seen on multiple installs
Seems like a recently introduced bug within a week or two

```
[100%] Linking C executable box64
CMakeFiles/box64.dir/src/main.c.o: In function `main':
/home/marlow/box64/src/main.c:1243: undefined reference to `pthread_atfork'
CMakeFiles/box64.dir/src/box64context.c.o: In function `NewBox64Context':
/home/marlow/box64/src/box64context.c:183: undefined reference to `pthread_atfork'
CMakeFiles/box64.dir/src/custommem.c.o: In function `init_custommem_helper':
/home/marlow/box64/src/custommem.c:1181: undefined reference to `pthread_atfork'
CMakeFiles/box64.dir/src/libtools/signals.c.o: In function `init_signal_helper':
/home/marlow/box64/src/libtools/signals.c:1421: undefined reference to `pthread_atfork'
collect2: error: ld returned 1 exit status
CMakeFiles/box64.dir/build.make:3750: recipe for target 'box64' failed
make[2]: *** [box64] Error 1
CMakeFiles/Makefile2:283: recipe for target 'CMakeFiles/box64.dir/all' failed
make[1]: *** [CMakeFiles/box64.dir/all] Error 2
Makefile:165: recipe for target 'all' failed
make: *** [all] Error 2
Compilation failed
```