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This makes the output considerably easier to use: it now highlights (in
red) matched tabs (instead of just listing the filenames), and adds
line numbers to both the tabs check and the space-less if check outputs.

The check-style exit status wasn't being propagated properly because
the loops were running in a subshell (and so the change the the
`errors` variable wasn't in the active command shell).  This fixes it
by running the greps in subshells and the loops in the main shell.

This also avoids the if(/for(/while( style check on
tests/CMakeLists.txt, since it *does* have if() statements with no space
that are producing error messages, but that is (acceptable) CMake style.

This adds a tool that checks style (currently just for tabs instead of
spaces in files under include/tests/docs) and produces a travis-ci build
failure if any problems are found.

Fixes #365. `sysconfig.get_config_var('SO')` already returns the correct
PYTHON_MODULE_EXTENSION, even for debug builds, so there is no need to
add anything else manually.

When run on windows in a venv, PYTHON_LIBRARY pointet to a non-existant
location in the virtual environment directory.

This has been fixed by testing if the path exists and, if not, trying
an alternative path, relative to the PYTHON_INCLUDE_DIR.

If the alternative path doesn't exit as well, an error will be raised.

Also:
- Update documented result variables
- Remove long-ago-deprecated variable

This prevented `DOC(...)` calls with 5 arguments to resolve correctly.