Ok, I deleted everything and started over. After all the cmake and make'ing, I still got to 100% and then the error (as shown above).
Did it again, but now with no LIB-path set to cmake. Exactly the same.
Ok... did it all again, this time setting LIB-path to libpython3.4m.so.
Yes, success!
Continued following the doc-file, downloaded the testplugin_v11.zip. Error.... not valid plugin file.... Argh..... what did i do wrong this time....

Downloaded the flightcrew-plugin, and that installed just fine! Puh....
Thanks for the help!