* Pass toolchain and user env variables to make invocation
* Rename configure --> make
* Add integration test coverage for make flag passing
This requires making the make_simple Makefile more realistic by
* using CXX and forwarding it to the wrapper;
* using CXXFLAGS instead of CXX_FLAGS and not overwriting its contents.
The macros utilise bazel "transitions" to set the `make` toolchain used
in the configure_make(), cmake() or make() rules to
a given make variant toolchain, e.g. preinstalled_nmake.
Note that the msvc constraint was removed from the
`exec_compatible_with` attribute of `preinstalled_nmake_toolchain` as
the condition is not actually met even when building with msvc. See
https://github.com/bazelbuild/bazel/issues/7730.
This will be tested in PR#729
* Added `targets` API to all existing build rules
* Updated examples
* Restore making the make toolchain always available.
* Add support for generator cmake parsing and setting CMAKE_MAKE_PROGRAM
* Cleaned up duplicate generator arguments
* Fixed cmake tests
* Updated docs
* Addressed PR feedback
* Fixed missing and incorrect generators
* Fixed `generate_args` name
