Configuration: nullability-completeness warnings suppressed

With a recent update of macOS systems (macOS Big Sur 11.2.3 and Xcode
12.4), there are many warnings when building C programs (for example the
simple program we compile to check the compiler, or some of the software
like `gzip'). It prints hundreds of warning lines for every source file
that are irrelevant for our builds, but really clutters the output.

With this commit, these warnings are disabled by adding
`-Wno-nullability-completeness' to the 'CPPFLAGS' environment
variable. This has also been added to the very first check of the C
compiler in the configure step.

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