R ld library path

added extra paths for LD_LIBRARY_PATH, LIBRARY_PATH and PKG_CONFIG_PATH

I think if this PR is accepted this one can be closed: https://github.com/hpcugent/easybuild-easyconfigs/pull/1150

@boegel what do you think?
can this ever be `lib`? I think it's hard to tell, so consider `lib` too?
```python
    'LD_LIBRARY_PATH': ['lib64', 'lib', 'lib64/R/lib', 'lib/R/lib'],
    ...
```

EasyBuild will only include `prepend-path` statements for directories which are actually there.