Recommend a standard coding style for meson.build files

It seems that there is no recommended standard for the coding style of meson.build files.

So in some projects the indentation is with two spaces, in other projects it's with tabs or four spaces, etc etc.

I think it's useful to have the same coding style in all projects, it makes contributing easier, it is also easier to copy/paste a snippet of code between projects.

Like the PEP 8 for Python: https://www.python.org/dev/peps/pep-0008/