* Extract shell fragments into a toolchain
Construct script in framework.bzl using special notation for calling
shell fragments, defining and referencing environment variables;
have the script converted from this special notation into a real script
(see README.md notes.)
The special notation is used to keep script construction in code still readable.
Tools scripts (cmake, ninja) were also converted.
* Fix reference to environment variables in examples
.. and reference it correctly $$var_name$$ so it is replaced further
* Review comments: documentation, explicitly mark functions from toolchain
- mark functions from shell toolchain as ##funname##, so that they
can not be mixed with usual shell functions
* Fix operating system name for windows in toolchain file
* Additional env vars for windows
* Correct cmake tool build
* Do not build ninja on Windows
* When run on Windows under msys, find utility from Window could "leak"...
into the script. Use heuristics to use /usr/bin/find if it exists.
Also, use pwd -W to get Windows-styled paths (for cmake)
* Extract cmake and ninja into a toolchain; provide defaults.
In the workspace-level rule registration function, default toolchain
implementations are registered for all platforms, which are calling
the cmake and ninja just by name, assuming they are preinstalled.
Also, the user can pass the custom toolchains in the initialization
function, they will be registered before the defaults.
* Do not run ninja build test for now without nested workspaces support
* Corrections after merge
* Always build ninja from sources for tests, correct initialization order
* Add example of using prebuilt artefact in the native tools toolchain.
* Review comments, have native_tool_toolchain#target, not label
+ improve documentation
* Correct ninja toolchain label
* Correct documentation in native_tools_toolchain.bzl
