Add a mechanism to add prefix/suffixes to configuration files

Right now, with configure_file() if the input: kwarg is not set, we just dump the entire configuration. This has the advantage of not having to maintain a duplicate list of defines (one in meson.build, another in config.h.in) and is less error-prone.

However, this leaves a gap: sometimes the config.h is mostly generated, but has a few static defines in it. We should implement a way to also add these to the generated config.h. There are two ways to implement this:

1. Add prefix: and suffix: kwargs to configure_file()

2. Add a new token type #meson_dump_configuration (or similar) that can be put in config.h.meson that will dump the configuration_data() object in that place

The latter has the advantage of being more flexible, and the former has the advantage of having all the defines in one place (in meson.build).