$< ignores order-only prerequisites

[Parke]

Hi bug-make,

It appears that if there are no normal prerequisites, $< will ignore
any order-only prerequisites.

This is not the behavior I expected, and I could not find
documentation describing this behavior.

Possibly relevant documentation:

https://www.gnu.org/software/make/manual/make.html#index-_0024_003c
https://www.gnu.org/software/make/manual/make.html#Prerequisite-Types

Below is a sample Makefile that demonstrates this unexpected behavior.

Thanks!

-Parke

----

$ cat Makefile

main: message foo bar

foo:  Makefile
bar:  |  Makefile

foo bar:
        @ echo $@ $<

message:
        @ echo
        @ echo expected output:
        @ echo
        @ echo foo Makefile
        @ echo bar Makeflie
        @ echo
        @ echo actual output:
        @ echo

$ make

expected output:

foo Makefile
bar Makeflie

actual output:

foo Makefile
bar