Makefile, include: makes conditional compilation more robust

Makefile

@@ -18,6 +18,8 @@ PREFIX ?= /usr/local
 
 # for conditionally compiling OS features
 OS != uname
+OS_INCLUDE != test -e include/$(OS).mk && printf 'include/$(OS).mk\n' \
+	|| include/None.mk
 
 # normalized prefix
 PREFIX_N != dirname $(PREFIX)/.
@@ -77,9 +79,8 @@ docs: docs/ build
 		"s/X\.X\.X/$$(git describe --tags --long | cut -d'-' -f1)/g")"; \
 		sed "s/$$original/$$title/g" <"$$file" >"build/$$file"; done
 
-
 # include OS feature libraries for compilation
-include include/$(OS).mk
+include $(OS_INCLUDE)
 
 .PHONY: rustlibs
 rustlibs: build/o/libgetopt.rlib build/o/libstrerror.rlib \