Use type cmp to test for cmp in Makefile

[dylanjgscott]

This PR will allow building on BSD based systems.

The --version option to cmp doesn't exist in BSD so cmp --version always returns non-zero status on BSD systems, causing make to fail.

Using type cmp instead of cmp --version will work on both GNU and BSD systems.

[cbbrowne]

Seems like a good change, with the caveat that I'm not utterly certain "type" works everywhere. Usually Linux uses Bash, but Debian and Ubuntu often are using Dash, a POSIX-compatible smaller shell

Note https://en.wikipedia.org/wiki/Type_%28Unix%29

It's also possible that a good command, per POSIX, would be "command -V cmp".

cbbrowne@cbbrowne ~/G/q/k/p/k/cbbrowne> command -V cmp
cmp is /usr/bin/cmp
cbbrowne@cbbrowne ~/G/q/k/p/k/cbbrowne> /bin/dash
$ command -V cmp
cmp is a tracked alias for /usr/bin/cmp
$ type cmp
cmp is a tracked alias for /usr/bin/cmp
$ which cmp
/usr/bin/cmp

That suggests that command -V, type, and which are all plausible choices.

[dylanjgscott]

I'm not certain that type will work absolutely everywhere either but it will at least work on Mac, GNU/Linux and BSD. I would guess that if type cmp doesn't work on any system then cmp --version won't work on that system either.

[fredizzimo]

What about cmp --help would that work on all platforms that has the cmp command?

[dylanjgscott]

No, unfortunately BSD doesn't have a --help option.

[fredizzimo]

I guess then we could do cmp $(ROOT_DIR)/Makefile $(ROOT_DIR)/Makefile which would actually test that the cmp command in itself compares two equal files as the same.

[dylanjgscott]

Yeah, that's a good idea. I've changed my PR to use cmp $(ROOT_DIR)/Makefile $(ROOT_DIR)/Makefile.

[fredizzimo]

@jackhumbert, This pull request has been open for a long time now, and I believe the implementation should work on all platforms now, so I think this is pretty safe to be merged.

[jackhumbert]

Thanks for the ping :) Thanks all!