This is a new FortranCInterface.cmake module to replace the previous
prototype.  All module support files lie in a FortranCInterface
directory next to it.

This module uses a new approach to detect Fortran symbol mangling.  We
build a single test project which defines symbols in a Fortran library
(one per object-file) and calls them from a Fortran executable.  The
executable links to a C library which defines symbols encoding all known
manglings (one per object-file).  The C library falls back to the
Fortran library for symbols it cannot provide.  Therefore the executable
will always link, but prefers the C-implemented symbols when they match.
These symbols store string literals of the form INFO:symbol[<name>] so
we can parse them out of the executable.

This module also provides a simpler interface.  It always detects the
mangling as soon as it is included.  A single macro is provided to
generate mangling macros and optionally pre-mangled symbols.