minor update to win64 warning messages to support toggling via env var

[zpangwin]

This adds support to allow users to export W_NO_WIN64_WARNINGS=1 for toggling off win64 warning messages (this new way is in addition to the file-based logic from an earlier commit and so should not break any existing setups).

This proposal and the advantages were previously discussed in the comments of issue #1730 but for convenience, those were:

1. Allows users to completely suppress the message (including echo statement from touched file) if they really want to

2. Allows to set an environment variable for a terminal session but not forever (or alternately could use .bashrc or equivalent for all terminal sessions)

3. Reduced number of I/O hits checking for a file (I know it is an extremely minor I/O hit but it's still technically a plus)

4. This also allows a user to temporarily turn the warning back on if they have created the file but wish to restore the warning temporarily such as for a single debugging session.

I think the actual changes should be pretty straight forward and keeping with the same coding style. But if there are any issues / questions / comments / etc, please let me know.

[austin987]

Thanks!