Implement doesCommandExist for Posix instead of linux

[gizmomogwai]

See https://stackoverflow.com/questions/762631/find-out-if-a-command-exists-on-posix-system
for reference.

[gizmomogwai]

This also includes osx on which i ran into the problem and tested the patch.

[BradleyChatha]

Thanks! I don't own a Mac so I can't test anything at all for it, much appreciated.

I haven't even started work on v0.13.0 yet, so I'll push this to dub as v0.12.1 for your convenience, I'll make sure to include this in the v0.13.0 changelog though.

[gizmomogwai]

thanks a lot ... it is not really critical for me, but good to have it also working on os x ...