Utilities/Sphinx: Add option to build 'singlehtml' format

Add SPHINX_SINGLEHTML to enable the Sphinx 'singlehtml' builder.
Restore --help-full option to output all help manuals

This option was removed during conversion to the reStructuredText
documentation.  Restore it.  Process documentation starting at
Help/index.rst so that all manuals are included in the output.
Merge branch 'doc-full-help' into release
Merge branch 'doc-singlehtml' into release
ExternalProject: Fix path to cmake in verify script

In commit v3.0.0-rc1~103^2~3 (ExternalProject: Reattempt download when
verification fails, 2014-01-15) a reference to ${CMAKE_COMMAND} was
added to generate a reference to the CMake command in a cmake script.
Escape the '$' so that the literal variable reference appears in the
script instead of writing the path to the current cmake.  This is
necessary when the path to CMake contains spaces or other characters
special to CMake syntax.
Merge branch 'ExternalProject-verify-cmake-var' into release