Index: share/mk/bsd.endian.mk
===================================================================
--- share/mk/bsd.endian.mk
+++ share/mk/bsd.endian.mk
@@ -15,6 +15,16 @@
 TARGET_ENDIANNESS= 4321
 CAP_MKDB_ENDIAN= -b
 LOCALEDEF_ENDIAN= -b
-.else
+.elif ${.MAKE.OS} == "FreeBSD"
 .error "Don't know the endian of this architecture"
+.else
+#
+# During bootstrapping on !FreeBSD OSes, we need to define some value.  Short of
+# having an exhaustive list for all variants of Linux and MacOS we simply do not
+# set TARGET_ENDIANNESS and poison the other variables. They should be unused
+# during the bootstrap phases (apart from one place that's adequately protected
+# in bsd.compiler.mk) where we're building the bootstrap tools.
+#
+CAP_MKDB_ENDIAN= -B	# Poisoned value, invalid flags for both cap_mkdb
+LOCALEDEF_ENDIAN= -B	# and localedef.
 .endif