Fix FileExists in Utils.cpp

Testing for st.st_mode & S_IFLNK doesn't work on OSX, because IFLNK and IFREG
share bits, so regular files have a nonzero result and appear to be links. It
seems safer and more readable to use the mode test macros, e.g. S_ISLNK and
S_ISDIR.

lib/common/Utils.cpp

@@ -346,9 +346,9 @@ bool FileExists(const std::string& rFilename, int64_t *pFileSize,
 	}
 
 	// is it a file?
-	if((st.st_mode & S_IFDIR) == 0)
+	if(!S_ISDIR(st.st_mode))
 	{
-		if(TreatLinksAsNotExisting && ((st.st_mode & S_IFLNK) != 0))
+		if(TreatLinksAsNotExisting && S_ISLNK(st.st_mode))
 		{
 			return false;
 		}