Make installed linux library symlinks relative to most versioned target.

[de-vri-es]

This PR does two things:

• It makes the symlinks to the installed library relative.
• It makes the most versioned target the real one, and the other ones symlinks.

The first one is to make it easier to package the software for a package manager. Often you want to "install" to a staging directory which is then compressed and turned into a system package. If the symlinks are absolute to that staging location, the installed package will have broken symlinks.

The second change is because it allows installing multiple versions without breaking the others. If the least versioned target is the real one, the more versioned targets will incorrectly be updated if someone installs a different version that overwrites libdxl_*.so.

[LeonJung]

@de-vri-es

This PR is very reasonable, but will be happened in next revision.
Since I want to let beginners in coding to make their examples easily,
I should think about which is the best way to let libdxl_xyz_x.so and libdxl_xyz_x.so.a.b.c files in the build folder so that they can use libdxl_xyz_x.so without modification.
Let's talk about this more.

anyway, thank you for your all kind advices
your help made DynamixelSDK with more perfection.
Please keep going your activities of suggestion.

I'll leave this here until when next revision comes with this PR

[de-vri-es]

No rush, thank you for looking at the PR. I'm very happy that you supply an open source SDK. That makes working with your motors so much easier compared alternatives with closed source SDKs.

I don't think that the symlinks have a negative impact on user experience by the way. On UNIX platforms, tools work the same both with symlinks and with regular files. So gcc main.c -ldxl_xyz_x works either way. Still, there's no need to hurry I think.

Again, thanks for the awesome open source SDK :)

[LeonJung]

@de-vri-es

As I said earlier,

I will throw a message and reopen issue when heres ready for talking about this for next release. :)