[anemitz]

This PR restores ROARING_BUILD_C_TESTS_AS_CPP and ROARING_BUILD_C_AS_CPP, including the external C function definitions needed when C callers link against a C++-built library.

Why care?

For it:

• The options already exist and are broken. They date from 2020 (Add C++ Namespaces (retry to trigger Appveyor) #253). Their stated purpose was that compiling C as C++ "is a useful test for catching certain errors" and enables C++ static analysis. They have never been in CI and have quietly broken: on master, g++ can't compile bitset.c as C++. A build option that doesn't build is worse than no option.
• The linkage part is needed for the option to be usable. A C++-built library that C programs can't reliably link against defeats the purpose of building the library.
• CI keeps it from breaking again. Without CI, these modes break silently.

Against it:

• Nobody seems to use the modes. I found no upstream issues about them, and they've been broken unnoticed.
• It found no real C bugs. The fixes it forced (the casts, the MM_PERM* constants) are C++ compatibility changes, not bug fixes in C.
• It adds ongoing cost.
  • Every new header inline function must use the macro.
  • Every push runs 8 more CI jobs.
  • There's a new shell script to maintain.
  • C++ builds export 45 extra symbols.
• MSVC still doesn't work. That's the toolchain where someone would most likely compile C as C++.

Validation and implementation notes

• GCC 15 and Clang 22 on Linux ARM64: all four static C/C++ library/test combinations and the shared C++ library with C tests pass, 28 tests per configuration (280 total).
• All 562 global C function symbols are present in the C++ static and shared libraries under both compilers. The C++ amalgamation also preserves all 562 symbols.
• GCC 13 on x86-64 Ubuntu 24.04, with the workflow's commands: the C++ library with C tests (static and shared) and the C++ library with C++ tests pass, 28 tests each, and all 578 C function symbols are present in the C++ static and shared libraries according to GNU nm. The ARM64 runs above compile out the AVX-512 code, which is how the _MM_PERM_ENUM error first reached CI.
• The linkage regression test and symbol comparison both fail against a library lacking container_iterator_next and container_iterator_prev.
• AppleClang: Debug shared build and seven focused tests pass; the Release x86_64 C linkage test passes under Rosetta, and the Release static C linkage test passes with LTO.
• Clang-format 17, YAML parsing, shell syntax, and symbol-checker error-path checks pass.

The C++ build emits 45 constant croaring_inline_* pointer symbols as implementation details to retain callable inline definitions without compiler-specific retention flags. Static-inline helpers are not retained this way.

[anemitz]

Perhaps the best course of action is to just remove these broken options since it's unclear if there's value in fixing this.