openmp/system: new recipe

[valgur]

Adds a new openmp/system package for a much more convenient and less bug-prone OpenMP support in recipes.

This avoids the need for listing the appropriate flags in every recipe using OpenMP, e.g. https://github.com/conan-io/conan-center-index/blob/master/recipes/lightgbm/all/conanfile.py#L123-L133

It relies on the default OpenMP provided by the compiler for compilers that ship with one (GCC and MSVC) and falls back to llvm-openmp from CCI for others (Clang and AppleClang).

transitive_headers=True, transitive_libs=True can be used to set the correct visibility for libraries that use #pragma omp in their public headers.

See #24577 for more context.

Requires

• llvm-openmp: match FindOpenMP.cmake output, add MSVC support #22353
• llvm-openmp: fix incompatibilities with official FindOpenMP, add v18.1.8, simplify patches, add MSVC support #24584

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 11 (fda846f782bf90dfa592b294bee0f26f1acc6fd7):

• openmp/system:
  All packages built successfully! (All logs)

---

Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 11 (fda846f782bf90dfa592b294bee0f26f1acc6fd7):

• openmp/system:
  All packages built successfully! (All logs)