[3.3b] Improved file comparison in preprocessor

[mefoster]

In the Ice 3.3 beta, the preprocessor compares file names to see if two files are identical. In the presence of relative file names and symbolic links, this isn't always reliable. The attached patch instead checks whether two files are really the same using "stat" and checking the inodes.

This works well on Linux, but I'm unfortunately not in a position to test on other operating systems.

MEF