cmake: drop `HAVE_C_FLAG_Wno_long_double` logic for ancient Apple gcc

The initial curl CMake commit introduced it in 2009-04-02 via
4c5307b45655ba75ab066564afdc0c111a8b9291. Suppressing a stray
`-Wlong-double` warning in `mprintf.c`. This was before Apple switched
to clang, and likely affected the Apple distributed GCC, version 4.2.1
at the time. It applied the workaround to CMake builds only, though
the issue seems build-tool agnostic. Yet, it was not suppressed or
reported for autotools builds.

For these reasons this logic seems obsolete and this patch drops it with
no replacement. It saves a feature detection for GCC builds for macOS.

In PR sub-commits I added (and reverted) in-source suppression. In case
it becomes necessary, that should fix it for all build tools.

Closes #16513

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