[CMake] Querying targets for Fortran module files & module file installation advice

[Zaak Beekman]

Hi all,

What's the best approach for handling cross-platform (i.e., MSVS, Mac,
 Linux) installation of Fortran module files associated with
libraries?

After searching the docs, I couldn't find any good and obvious way to
handle installation of Fortran module files associated with library
targets. These are compiler specific, and, perhaps a better approach
might be to ship all executable code compiled into libraries and put
inside submodules, and ship a free/open parent module with interface,
type, class, constant, etc. definitions.

But, at any rate, it would be nice if there were a better way to
install Fortran module files besides something like:

```
set(CMAKE_Fortran_MODULE_DIRECTORY "${CMAKE_BINARY_DIR}/include")

...

install(DIRECTORY ${CMAKE_Fortran_MODULE_DIRECTORY} DESTINATION
${CMAKE_INSTALL_INCLUDE_DIR})
```

In particular, it is hard to know which .mod and .smod files will be
produced ahead of time without parsing the source, or manually
enumerating them. One can use the approach outlined above, but a few
problems and drawbacks remain:

1. With IDEs like MSVS, I always seem to end up getting an additional
directory installed under ${CMAKE_INSTALL_INCLUDE_DIR} with the name
of the build configuration (e.g., `debug`)
2. Utility/test modules may have generic names (e.g. assertions.mod)
that probably should not get installed and will very likely lead to
name  clashes with consuming projects
3. Lack of standard definition of module file format and Fortran ABI
makes portability dicey, at least until ISO_Fortran_BINDING is widely
available and adopted

It would be great if there were a target property that could be
queried for associated module and submodule files. In addition
additional "it just works" logic around installing Fortran targets and
how modules are handled would be really nice. This obviously isn't an
easy/trivial question to answer, but being able to query targets for
module file build artifacts would be a great starting point.

In the mean time, I think my only acceptable options to handle IDEs
like MSVS is to use the `SCRIPT` signature of `install()` where I can
use `file(GLOB_RECURSE ...)` to find module files and strip out any
build configuration specific directory structure. (Or at least adopt
my own logic and conventions for this...)

Any tips or advice would be very appreciated!

Thanks,
Zaak