-# omitted =1 is assumed.
-
-PREDEFINED = __linux__ __unix__ FORMAT(x,y,z)= PGM_FUNC(x)=x
+# omitted =1 is assumed. To prevent a macro definition from being
+# undefined via #undef or recursively expanded use the := operator
+# instead of the = operator.
+
+PREDEFINED = __linux__ \
+ __unix__ \
+ FORMAT(x,y,z)= \
+ PGM_FUNC(x)=x