Idea: MAKE_SILENCEWARNINGS variable silences make warnings

[David A. Wheeler]

Problem:

Make warnings are generally useful, but in special cases they can be spurious.
It'd be nice to be able to control them.

Solution:

Create a new make special variable "MAKE_SILENCEWARNINGS".
By default it is not set (empty). If it's non-empty (true),
then make does not print make warnings.  Make errors are still printed, and
commands (which can generate messages) are unaffected.

It's expected that MAKE_SILENCEWARNINGS would typically be used
as a target-specific or pattern-specific private special variable. E.g.,

weird-thing: MAKE_SILENCEWARNINGS = true

--- David A. Wheeler