CMAKE generators list

[hinell]

From cmake-generators.7 it follows, that:

... command-line tool --help output lists available generators on the current platform

but currently we only have a hard-wired list of available generators: https://github.com/zsh-users/zsh-completions/blob/b131623ee4c46aa9a781213a3ba0e4cfd020b810/src/_cmake#L470-L486

I propose to make a list platform-dependent. Any thoughts?