serial: Add Darwin/macOS support for custom and >230400 baudrates

This change is based on the patch proposed by Denis Ahrens in
https://review.coreboot.org/c/flashrom/+/67822

Change-Id: I3e6b88d2b4c2a130b16456752681fd9f807bf6f0
Signed-off-by: Peter Stuge <peter@stuge.se>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/70571
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73480
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/meson.build b/meson.build
index 7a91300..fa08c93 100644
--- a/meson.build
+++ b/meson.build
@@ -100,6 +100,8 @@
 
 if host_machine.system() == 'linux'
   custom_baud_c = 'custom_baud_linux.c'
+elif host_machine.system() == 'darwin'
+  custom_baud_c = 'custom_baud_darwin.c'
 else
   custom_baud_c = 'custom_baud.c'
 endif