Bind an environment variable on another

[xdelaruelle]

Following an old discussion with OpenHPC dev team at SC18, it is sometimes desired to bind an environment variable onto another to avoid duplicates definition commands in modulefile. For instance with DYLD_LIBRARY_PATH and LD_LIBRARY_PATH, or CPATH and INCLUDE.

It would be expected that any change on a variable would be applied on every (0 to N) variables bound to it.

Some design notes would be drafted to determine if a new configuration option or a new modulefile command should be introduced.