Implement build of Doxygen documentation

If CLHEP_BUILD_DOCS is set, search for the Doxygen executable, failing
if it is not found. Convert doxygen configuration file to a
CMake-configurable file, with expansion variables to set version,
output directory and HTML header/footer and stylesheet.

Add custom command/target pair for running doxygen with configured input
file. Command depends on main CLHEP target because headers must have
been copied into build directory before doxygen can run.

Install doxygen output to hardcoded share/CLHEP/doc directory under
install prefix.

Make clhep_latex macro a no-op and comment out recursion into doc
directories for the packages that have this subdirectory. This avoids
installing odd documents directly into the install_prefix...
TODO : Install these documents in the right place, and convert TeX to
Markdown so these can be integrated with Doxygen.