Recipes for fpm functionality

[awvwgk]

We need more How-To guides in the documentation pages. Here are some ideas:

• building an fpm application with MPI
  • --compiler keyword for MPI wrapper
  • external-modules manifest keyword
  • hybrid MPI via preprocessor
• linking against system libraries
  • link manifest keyword
• including C code in a project
  • header placement
  • include-dir manifest keyword