Fix incorrect name casing in znn.lang

See merge request !139

add new generated shell completions and install them

See merge request !137

Fix Clang build and incorrect _WIN32 macros

See merge request !138

Clang complains of getpid as unknown idenfier.  It’s declared in
POSIX’s unistd.h (unsupported on Windows) and process.h for Windows.
Include based on platform.

 _WIN32 is the official macro [1] while WIN32 is
 incorrectly used in code.

[1]: https://stackoverflow.com/a/2348852/183120

Also fix the system-wide config directory when PREFIX is not /usr.

Fix MSYS2 / MinGW build

See merge request !136

* Fix WIN32 macro to _WIN32; latter is the official one [1]
* Pass -ldl only for non-Windows platforms
  - dl{open,close} is unavailable on Windows and thereby MinGW

[1]: https://stackoverflow.com/a/2348852/183120

update manpage

See merge request !135

- names of the options are bold
- option's arguments are italic
- environment variables are italic