Avoid removing real files when case-insensitive

Case-insensitive filesystems (such as OS X uses by default) have various
important things such as parser.h removed during the add-names step;
check for this situation and avoid doing the removal/link if Parser.h is
already a real file.
Merge pull request #1 from ziz/case-insensitivity

Avoid removing real files when case-insensitive (MacOS X is like this by default)