Build fails on arch

[JamesCrompton]

Chugs away happily until this:

[ 60%] Building CXX object src/CMakeFiles/romlib.dir/serial.cpp.o
.../executor/src/serial.cpp:192:19: error: field ‘state’ has incomplete type ‘termio’
  192 |     struct termio state;
      |                   ^~~~~
.../executor/src/serial.cpp:192:12: note: forward declaration of ‘struct termio’
  192 |     struct termio state;
      |            ^~~~~~
.../executor/src/serial.cpp: In function ‘Executor::OSErr serset(Executor::LONGINT, Executor::INTEGER)’:
.../executor/src/serial.cpp:539:19: error: aggregate ‘termio settings’ has incomplete type and cannot be defined
  539 |     struct termio settings;
      |                   ^~~~~~~~
.../executor/src/serial.cpp: In function ‘Executor::OSErr serxhshake(Executor::LONGINT, Executor::SerShk*)’:
.../executor/src/serial.cpp:702:19: error: aggregate ‘termio settings’ has incomplete type and cannot be defined
  702 |     struct termio settings;
      |                   ^~~~~~~~
.../executor/src/serial.cpp: In function ‘Executor::OSErr setbaud(Executor::LONGINT, Executor::INTEGER)’:
.../executor/src/serial.cpp:775:19: error: aggregate ‘termio settings’ has incomplete type and cannot be defined
  775 |     struct termio settings;
      |                   ^~~~~~~~
make[2]: *** [src/CMakeFiles/romlib.dir/build.make:2941: src/CMakeFiles/romlib.dir/serial.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:976: src/CMakeFiles/romlib.dir/all] Error 2
make: *** [Makefile:166: all] Error 2