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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