Script for analyzing include dependencies.

Script for analyzing include dependencies.

Add a Python script plus some CMake code to run it that analyzes include
file dependencies.  The script can also generate different kinds of
include dependency graphs (this implementation dates from a time when
some versions of Doxygen didn't do this properly, and the script still
has some features that Doxygen doesn't have).  Some parts of the script
contain hard-coded stuff related to the Gromacs source tree layout or
the Doxygen guidelines, but the core dependency analysis should be quite
general.

There is a lot to improve in the script (in addition to missing
features, docstrings and comments are sparse, and some parts could be
better structured), but it is already usable.

Fix a few issues that produce multiple warnings from the script, and one
issue found by the script (in helpformat.h).

Related to #638 and #907.

Change-Id: I22a6d6c0818f21c828f1e7d6beb1fdada39273d2