configure: Properly check for availability of -Wno-foo warning flags

For some weird reason gcc does not check if the -Wno disabling variants
of warning flags match existing warning flags. Instead it swallows them
silently. That is, unless other warning or error messages are generated,
because then - for some even more bizarre reason - a complaint about the
unknown disable warning flag is issued along with the error or warning
message.

Thus to check for the availability of a warning disabling option, one
needs to check for the enabling variant instead and then add the
disabling variant to CFLAGS.

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