Fix broken pkg-config files

Currently using dune by installing it has several issues:

• 'pkg-config' support is broken: While 'module.pc' files are installed, using 'pkg-config' to determine the flags needed to build against dune are incomplete.
• Documentation in 'dunecontrol': 'dunecontrol --help' does not tell you anything about how to install and how to specify an installation prefix.
• Documentation on https://dune-project.org/doc/installation
  • While '-DCMAKE_INSTALL_PREFIX=...' in contained in a code example it's not explained what it does.
  • In contrast to this documentation you have to specify the option file when calling 'dunecontrol make install'
  • The documentation still mentions 'autogen'

All of this makes it hard to build code based on dune if you are neither using the dune build system nor cmake. Pkg-config would be a way out but is currently broken.