use rpath for dependent libs

[bebehei]

Fixes #437

Before merging this PR: Why is an additional runpath not needed for the plugins, although these require globalshortcut and xdg directly?

[ManuelSchneid3r]

Globalshortcut should not need it. Since it has no dependencies. Well I have no clue why the extensions dont need it. Maybe RPATH gets inherited by the linking elf.

[bebehei]

bebe@ubuntu17:~$ ldd /usr/local/lib/albert/plugins/libfirefoxbookmarks.so | grep found
        libalbertcore.so => not found
        libxdg.so => not found

But running it with albert spits no errors and it works fine.

[ManuelSchneid3r]

damn this pr is screwed up now. well ill set this for the plugins too. thank you for your investigatuions benedikt. merged manually