test/run_tests: extend support for more operating systems

Not all systems recognise LD_LIBRARY_PATH, and we therefore need to
set other environment variables.

Currently set:

- LD_LIBRARY_PATH (Linux and ELF HP-UX)
- DYLD_LIBRARY_PATH (MacOS X)
- LIBPATH (AIX, OS/2)

More can be added as the need arises.

Fixes #146