Problems building on OpenSuSE Leap 15.1

[smcameron]

Build instructions could use updating for OpenSuSE Leap 15.1. Ran into the following:

1. Needed to install pkg-config and pkg-config_files.
2. Needed to set pkg-config-path environment variable (to what? Need to find this out.)
3. All references to lua 5.2 needed to be changed to lua 5.3. Note, we only compiled snis_client (via "make bin/snis_client") which shouldn't need lua at all. I don't know if the differences between lua 5.3 and lua 5.2 are significant enough to break things. I normally use lua 5.2 on my systems.
   Changes to Lua 5.3 -- at a quick glance I didn't see anything that I think will be problematic.

[smcameron]

Added this info to the build instructions (still needs work): c520e4e