To build a Universal Binary CHICKEN on Leopard that also runs on Tiger you do:

export MACOSX_DEPLOYMENT_TARGET=10.4
make PLATFORM=macosx ARCH=universal

(Thanks to Heinrich Taube)

What worked for me on 10.14.2

--- Makefile.macosx.bak	2019-01-18 21:32:04.000000000 -0800
+++ Makefile.macosx	2019-01-18 21:34:18.000000000 -0800
@@ -28,9 +28,9 @@
 
 # platform configuration
 
-XCODE_DEVELOPER ?= /Applications/Xcode.app/Contents/Developer
-XCODE_TOOL_PATH ?= $(XCODE_DEVELOPER)/Toolchains/XcodeDefault.xctoolchain/usr/bin
-C_COMPILER ?= $(XCODE_DEVELOPER)/usr/bin/gcc
+XCODE_DEVELOPER ?= /Library/Developer
+XCODE_TOOL_PATH ?= $(XCODE_DEVELOPER)/CommandLineTools/usr/bin/
+C_COMPILER ?= $(XCODE_TOOL_PATH)/gcc
 ARCH ?= $(shell sh $(SRCDIR)/config-arch.sh)
 
 # commands