build: optionally skip external warnings

Add an option to `./configure` to suppress compilation warnings from
external headers. The option is off by default (no change in behavior,
show warnings from external headers).

This option is useful if e.g. Boost or Qt is installed outside of
`/usr/include` (warnings from headers in `/usr/include` are already
suppressed by default) and those warnings stand in the way of compiling
Bitcoin Core with `-Werror[=...]` or they just clutter the build output
too much and make our own warnings hard to spot.

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