Install location of mod files for generated bindings

[havogt]

When the user has Fortran enabled, the bindings will be build which will produce a .mod file. This module file will be placed in the CMAKE_CURRENT_BINARY_DIR (default).

If the user will try to modify this directory (with property Fortran_MODULE_DIRECTORY) there are currently a few limitations:

• the include directory attached to the target via BUILD_INTERFACE will still point to CMAKE_CURRENT_BINARY_DIR, i.e. the mod file will not be found (the directory has to be specified manually by the user)
• we don't specify a directory with INSTALL_INTERFACE as we (the library) cannot know where the user wants to place it...