output_required_files
---------------------

Deprecated.  Approximate C preprocessor dependency scanning.

This command exists only because ancient CMake versions provided it.
CMake handles preprocessor dependency scanning automatically using a
more advanced scanner.

::

  output_required_files(srcfile outputfile)

Outputs a list of all the source files that are required by the
specified srcfile.  This list is written into outputfile.  This is
similar to writing out the dependencies for srcfile except that it
jumps from .h files into .cxx, .c and .cpp files if possible.
