Only set WARNS if not defined

This would allow interested parties to do experimental runs with an
environment set appropriately to raise all the warnings throughout the
build; e.g. env WARNS=6 NO_WE

Only set WARNS if not defined

This would allow interested parties to do experimental runs with an
environment set appropriately to raise all the warnings throughout the
build; e.g. env WARNS=6 NO_WERROR=yes buildworld.

Not currently touching the numerous instances in ^/tools.

MFC after: 1 week

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