VMS: add a note about DECC$* logical names

These logical names are used to make the C RTL change certain
behaviors, sometimes to make it act more like Unix.  While they can
make life easier in some cases, they can be disruptive as well.  When
building and testing OpenSSL, the latter is the case, so we ask people
to avoid using them.

Reviewed-by: Tim Hudson <tjh@openssl.org>