GCC 5 mangles some symbols, such as "plugin_globals" ->
"plugin_globals[abi:c++11]" (or "_Z14plugin_globalsB5cxx11"), even
when declared with 'extern "C"'. This breaks plugins, in particular.
- script-syntax.py now exits with 1 instead of silently catching the
error if lua or ruby are missing
- index paths are absolute (from the repo root)
- less strong suggestion for third-party packs
- re-added lua-example and test-perlin doc
- corrected and clarified Compile some more