1. Call the version extraction script directly instead
of through a Python returned from `find_program()`.
2. We don't need to use `meson.project_source_root()` as
`find_program()` will search relative to the current meson.build
script.
3. Lower the required version back to `>= 0.37.0`, and modify
some things to get rid of warnings with this version selected.
The use of `summary()`, `dict`, and positional arguments in
`pkgconfig.generate()` generate warnings with this version so
remove `summary()` and dict()`, also pass keyword arguments to
`pkgconfig.generate()`.
The following features from the CMake build have been implemented:
* Library version retrieved from unity.h.
* Extension support.
* Library, header, and package configuration file installation.
This commit is entirely based on existing work by Owen Torres.
