Add sections to the cmake-buildsystem(7) manual and cross-reference
them with relevant variables and target properties.  This avoids
duplicating the information and allows it to be more detailed.

Portions of these documents contain duplicate content generated from
preprocessor macros in the original builtin documentation source.
Factor the common parts out into .txt files using reStructuredText
include directives and substitutions to avoid duplication.

Run the convert-help.bash script to convert documentation:

 ./convert-help.bash "/path/to/CMake-build/bin"

Then remove it.