Include names of include files in the hash again

Include names of include files in the hash again

This is essentially a revert of 5908e656 and 9cffdc65 (a partial fix of
the problem). The idea of pull request #88 ("preprocessor mode: enable
using identical header in different paths") did not work out well in
practice since it broke handling of dependency files. See the new "-MMD
for different ..." test cases which fail without the revert.

Fixes #134. Reverts #88.