define platform-identification CPP macros

Makefile.conf

@@ -87,6 +87,7 @@ endif
 # Are we wiping this on purpose?
 LDLIBS=
 
+# Define compiler.
 ifeq ($(BUILD_FOR),OS/390)
     CC=xlclang
 else ifeq ($(BUILD_FOR),Linux)
@@ -95,6 +96,16 @@ else ifeq ($(BUILD_FOR),Darwin)
     CC=clang 
 endif
 
+# Define platform-identification feature macros.
+ifeq ($(BUILD_FOR),Linux)
+    CPPFLAGS+= -DUNIX 
+else ifeq ($(BUILD_FOR),Darwin)
+    CPPFLAGS+= -DUNIX -DDARWIN 
+else ifeq ($(BUILD_FOR),OS/390)
+    CPPFLAGS+= -DZOS 
+endif
+
+# Add platform-specific compilation options.
 ifeq ($(BUILD_FOR),OS/390)
     CFLAGS= -Wno-gnu-include-next -qnosearch -I$(HERE) -I.
     CFLAGS+= -I/usr/include