MacOS X does support the tcsetattr serial port support supposed by

sqUnixSerial.c.  So instead of using the null support that was in
sqMacSerialPort.c, update the Mac builds to use sqUnixSerial.c, and
rename the mac's file to platforms/Cross/plugins/SerialPlugin/sqNullSerialPort.c

THIS IS UNTESTED!  In  particular the code in sqUnixSerial.c (dubiously IMO)
does not use serialPortBaseName, and instead makes an assumption that
serial ports are as defined by serialPortBaseNameDefault, i.e. are in the range
/dev/tty50 to /dev/tty59.  It would be great if
a) someone tested the code on MacOS X
b) the naming convention were better.
On Mac OS X (see e.g. https://software.intel.com/en-us/setting-up-serial-terminal-on-system-with-mac-os-x),
the serial ports are /dev/tty.NAME., e.g. /dev/cu.Bluetooth-Incoming-Port,
/dev/cu.BoseQuietControl30-SPPD, etc.

platforms/iOS/plugins/SerialPlugin/Makefile

@@ -1,7 +1,6 @@
-ifeq ($(VM),Pharo)
-EXTRALIBS:=-undefined dynamic_lookup
+#EXTRALIBS:=-undefined dynamic_lookup
+SRCDIRS:=../../platforms/unix/plugins/SerialPlugin
 INCDIRS:=../../platforms/Cross/plugins/SerialPlugin
-LIBSRC:=sqMacSerialPort.c
-endif 
+LIBSRC:=SerialPlugin.c sqUnixSerial.c
 
 include ../common/Makefile.plugin

platforms/unix/plugins/SerialPlugin/sqUnixSerial.c

@@ -1,6 +1,4 @@
-/* sqUnixSerial.c -- Unix serial support
- * 
- * Last edited: 2011-03-14 14:01:56 by piumarta on emilia.ipe.media.kyoto-u.ac.jp
+/* sqUnixSerial.c -- Unix (including untested MacOS X) serial support
  */
 
 #include "sq.h"
@@ -20,7 +18,11 @@
 
 #define PORT_NAME_SIZE 64
 
+#if __APPLE
+static const char serialPortBaseName[]		= "/dev/cu.";
+#else
 static const char serialPortBaseName[]		= "/dev/tty";
+#endif
 static const char serialPortBaseNameDefault[]	= "/dev/ttyS0";
 
 /* stopBits	0=1.5, 1=1, 2=2 */