Using ".dep" for the suffix on header dependency files in the obj directory.
Using ".dep" for the suffix on header dependency files in the obj directory.

Formerly I used ".i", but "alextingle" on HN observed that that's the suffix
for pre-processed C source.  That shouldn't pose any actual problem, since I
don't expect any other process to generate .i files in the obj directory, but
nevertheless I'm changing the suffix to ".dep" anyway to avoid even the
appearance of any potential problem.

chkoreff/Fexl,chkoreff/Fexl