Fixes regressions introduced in previous merges

Fixes a few issues introduced in recently merged pull requests.

https://github.com/sass/libsass/pull/441: `resolve_relative_path` would always return lowercase paths on windows, which will not work if one would deploy the files to a linux server (or any other case sensitive filesystem). Implemented a better and IMO correct way to handle this. Also introduced a preprocessor directive `FS_CASE_SENSITIVE` so that the behaviour could be overruled on compile time.

https://github.com/sass/libsass/pull/440: `sourceMappingURL` is always added even if `source_map_file` was not set to anything at all.