Few Makefile improvement

[gportay]

Hello,

I am using the target user-install for the last years as a convenient target to install projects to my home directory; mostly because I do not have enough privileges to install some development tools on some machines.

This PR consists of three commits:

• fix the installation of the bash completion script to ${completionsdir} using $(PREFIX)
• add COMPLETIONSDIR if the fix above is not enough for the package maintainers
• add user-install target to install files to ~/.local prefix

I hope this PR will be useful for developers.

Regards, Gaël

[gportay]

ping?

[joufellasfl]

Thanks @gportay , this is neat, pr is now merged.