Fix CMake C99 complex detection cache pollution

[brtnfld]

Remove premature flag assignment and derive H5_HAVE_C99_COMPLEX_NUMBERS
deterministically from cached type sizes instead of caching the flag.
Fixes re-configuration failures and eliminates cache pollution.

Also fixes typo checking _FCOMPLEX twice instead of _LCOMPLEX.

Fixes #6051

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Important

Fix C99 complex detection in ConfigureChecks.cmake by removing premature flag assignment and fixing MSVC type check typo.

• Behavior:
  • Remove premature assignment of H5_HAVE_C99_COMPLEX_NUMBERS in ConfigureChecks.cmake.
  • Derive H5_HAVE_C99_COMPLEX_NUMBERS from cached type sizes instead of caching the flag.
  • Fixes re-configuration failures and eliminates cache pollution.
• Bug Fixes:
  • Fix typo in MSVC complex type check: check _LCOMPLEX instead of _FCOMPLEX twice.

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