Python: fix installation with non-standard include path target of libyang
Python: fix installation with non-standard include path target of libyang

When installed outside of default `/usr` prefix, the build fails because
`setup.py` does not respect CMake's library locations. I have no idea
whether it respects non-standard compilers etc, but this one was an
issue that I hit:

```
x86_64-pc-linux-gnu-gcc -pthread -fPIC -I/home/jkt/temp/py-3.6/include -I/usr/include/python3.6m -c /home/jkt/work/cesnet/gerrit/CzechLight/cla-sysrepo/submodules/dependencies/libnetconf2/python/netconf.c -o /home/jkt/work/prog/_build/libnetconf2/GCC_8_1_NETCONF-Debug/python/build/temp.linux-x86_64-3.6/home/jkt/work/cesnet/gerrit/CzechLight/cla-sysrepo/submodules/dependencies/libnetconf2/python/netconf.o -Wall -I/home/jkt/work/prog/_build/libnetconf2/GCC_8_1_NETCONF-Debug/python-I/opt/nc/include -DNC_ENABLED_SSH -DNC_ENABLED_TLS
In file included from /home/jkt/work/cesnet/gerrit/CzechLight/cla-sysrepo/submodules/dependencies/libnetconf2/python/netconf.h:25,
                 from /home/jkt/work/cesnet/gerrit/CzechLight/cla-sysrepo/submodules/dependencies/libnetconf2/python/netconf.c:21:
/home/jkt/work/cesnet/gerrit/CzechLight/cla-sysrepo/submodules/dependencies/libnetconf2/python/../src/session_client.h:22:10: fatal error: libyang/libyang.h: No such file or directory
 #include <libyang/libyang.h>
          ^~~~~~~~~~~~~~~~~~~
```

The end result requires a `LD_LIBRARY_PATH` override because RPATH
information is getting lost as well, but hey, this is an improvement. It
would have been nice if all CMake settings were respected, but that's
outside of my experience.

CESNET/libnetconf2,CESNET/libnetconf2