Adjust Makefile to work with macOS

[dljsjr]

This PR makes two small tweaks to the Makefile for a slightly smoother experience on macOS:

• macOS ships with GNU make, not BSD make, but it doesn't ship any GPLv3 GNU tools. That means that it's stuck on GNU Make 3.81. While ::= and := are defined as equivalent in the GNU Make manual, GNU Make 3.81 only supports the form :=. This saves users from having to install gmake from homebrew.
• macOS uses BSD install, and BSD install uses positional args of the form file1 ... fileN directory instead of the -t|--target flag to specificy directory. The fix here is introducing a conditional.

[tim-janik]

* macOS uses BSD `install`, and BSD `install` uses positional args of the form `file1 ... fileN directory` instead of the `-t|--target` flag to specificy `directory`. The fix here is introducing a conditional.

Thanks for the PR!
Do you have a link to the man page for install on macOS? I'd rather find a syntax that works for both systems.

[tim-janik]

Please read through https://developercertificate.org/ and add a Signed-off-by line to your commit message.

This should probably be added to the README under some Contributions header or similar...

[dljsjr]

* macOS uses BSD `install`, and BSD `install` uses positional args of the form `file1 ... fileN directory` instead of the `-t|--target` flag to specificy `directory`. The fix here is introducing a conditional.

Thanks for the PR! Do you have a link to the man page for install on macOS? I'd rather find a syntax that works for both systems.

This seems to line up with the actual output of man install on macOS: https://ss64.com/mac/install.html

Looking at the Linux man page for install, it looks like it has a similar form where it takes a list of sources and a final destination directory as positional arguments: https://man7.org/linux/man-pages/man1/install.1.html (I'm looking at the 2nd POSIX-y form).

Additionally, from both man pages, it looks like -c is ignored/the default behavior, so the invocation could likely just be install jj-fzf "$(bindir)"

[dljsjr]

I just force-pushed an amended commit with the "unified" install invocation, it works under both my macOS and Ubuntu dev environments.