Hmm, I found it very straight forward on my Ubuntu 16.04LTS.
I initially installed what ever version of lazarus using apt. That solved all the dependencies for me. I then used apt to uninstall Lazarus, FPC-source and finally FPC then I grabbed the 1.8rcX debs, manually installed them, first FPC, the FPC-sources, then Lazarus.
And it all worked. That was on my laptop. On the desktop, I started with a totally clean install, again with the 1.8rcX deps, this time I let apt deal with the dependencies. Again, it all worked. I don't understand how it was so easy for me but is, apparently a problem for so many other people. Only "secret" I can think of is to do it in the right order. Thats important. I found lots of problems occur on the mac if you stray from the path even a little bit. (I doc'ed that on the mac Install Wiki page, I hope people believe me.)