Documentation for environment variables that control the default
generator selection:
* CMAKE_GENERATOR
* CMAKE_GENERATOR_INSTANCE
* CMAKE_GENERATOR_PLATFORM
* CMAKE_GENERATOR_TOOLSET
While we already support `VERBOSE` environment variable and
`CMAKE_VERBOSE_MAKEFILE` cached variable, add `-v` and `--verbose`
command line options to be able to activate verbose output directly from
CMake's build tool mode command line.
Also make `msbuild` honor the verbosity setting. `xcodebuild` still
doesn't honor the verbosity setting as it will need a policy added
and reworking of cmGlobalGenerator and cmsys to support
multiple command invocation.
In each environment variable doc page, insert an opening
paragraph (ENV_VAR.txt) to say that this is an environment
variable, and provide a link to the cmake-language section
on environment variables.
Add documentation for both the CMake variable and environment variable
of this name pattern. Update references to these names to link to their
documents. Clarify the pattern used to construct their names.
While we already support `cmake --build . -- -j`, the options after `--`
are specific to the native build tool. Add new options `--parallel
[<N>]` and `-j [<N>]` to abstract this and map to the proper option
for the native build tool.
