Define directory for Fortran's .mod files

[Jeff Johnson]

See discussion and longer introduction at
https://lists.linux-foundation.org/pipermail/fhs-discuss/2011-May/000018.html

Fortran compilers generate .mod files (when compiling source files which
contain modules), which are similar to precompiled C header files. They .mod
files should be at a defined location to make it simple to use compile
programs.

Linux distributions seem to be a bit lost where to place those files; one finds
several choices, including /usr/include, in some hidden-package-dependent
directory, somewhere in a subdirectory of /usr/lib, or even not shipping them
at all to avoid the issue.

Contrary to C header files and similar to precompiled C header files, the files
are in general compiler dependent, i.e. they should resist in a
compiler(-version) dependent subdirectory of the to-be defined directory.

A possible choice would be /usr/{local/,}lib/finclude/$compilername/
such as /usr/lib/finclude/gcc/x86_64-unknown-linux-gnu/4.7/