Remove Fortran interface files from the repo

The main change of this PR is that all Fortran interface files are removed from the repo and generated automatically (if needed) when running make.

My original solution from 10 months ago was quite complicated for the reasons described in 8f44a32f.

To make the generation of the files more transparent, I made a decision that the files should be generated in the maintainer mode only (i.e. when the library is configured with --enable-maintainer-mode). This means that once/if this MR is merged, it will not be possible to build develop with the Fortran interface enabled in non-maintainer mode. It won't be possible to create a dist tarball (make dist) either.

Note that this does not affect cases when building with --disable-maintainer-mode from the release tarballs/branches: if the automatically generated Fortran interface files are there, make will treat them as regular source files, without any attempts to update/regenerate them.

Also, the automatically generated files are not removed with make distclean because they are supposed to be in the dist. Please, use the standard make maintainer-clean if you want to remove them.

Additionally, this MR fixes the formatting and gets rid of several compiler warnings.