Improve implementation of install-missing-packages

[0mp]

Summary

Improve implementation of install-missing-packages

As suggested by mat@ in D23484, let's use a proper if statement here instead of
short-circuits operators.

Test Plan

Run make install-missing-packages in:

• a port which has all the dependencies already installed
• a port which is still missing some dependencies

Try all the above scenarios with the default SU_CMD and with SU_CMD= /usr/local/bin/sudo -E sh -c.

[mat]

Mk/bsd.port.mk

4336

Ah, no, I knew I saw something I did not like. Quote what you pass to echo.

$$(${ECHO} "$${_dirs}"

[0mp]

Mk/bsd.port.mk

4336

I should also use "ECHO_CMD" instead, right?

[mat]

Mk/bsd.port.mk

4336

yes

[0mp]

Thanks, @mat! :)))