If {ARCHIVE,LIBRARY,RUNTIME}_OUTPUT_DIRECTORY is set with a genex then
do not add the per-config subdirectory on multi-config generators.
This will allow projects to use $<CONFIG> to place the per-config
part of the directory path somewhere other than the end.
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.