[autorandr-launcher] Add makefile & create macro to enable local path of autorandr

[horstmannmat]

Hi, this PR adds some improvement to the code;

• adds the possibility of the user compiling it locally (with makefile), and using its local autorandr path, It defines a macro that the makefile can overwrite for a local Path.
• Also changes ar_launch to fit in the new C standards, and ar_log to use attribute
• Adds a check for the return of the execution, we never knew if it was running fine without this check

The internal makefile can be a readme explaining the process, it is up to the owner to define the best

[UPDATE] In fact, since it was assuming that the path would be always the "/usr/bin", but if for any reason it was different. Let's suppose that DESTDIR=~/ PREFIX=.local/ make install TARGETS='autorandr launcher' So I added also in the general makefile to use the user inputs.

The launcher is an alternative to the "udev/systemd setup" that is more stable for some users

The assumption of the PATH, combined with no error checking when executed could be the root cause of why udev/systemd is more stable for some user

Closes #209

[horstmannmat]

cool thing is that this resolves #209

[phillipberndt]

lgtm, thanks!