fix: rename gnu→gnu_omp variant and add stdlib(c) for Intel glibc

[gunnar1987]

Summary

Fixes two reported build issues:

1. GNU/OMP variant resolves to MPI at install time

The gnu variant's build string (gnu_h…) is a prefix of gnu_mpi_h…, so conda install "runner=*=gnu_*" matches both variants. Renamed gnu → gnu_omp to make the globs unambiguous.

2. Intel variant linked to wrong glibc

${{ stdlib('c') }} was only in the GNU branch of the compiler conditional. The Intel build:

• Used system glibc 2.34 (Alma 9 Docker image) instead of the conda-forge sysroot
• Produced a package with no __glibc constraint in the metadata

Fix: moved stdlib('c') outside the conditional and set c_stdlib_version=2.28 for Intel (required by ifx_linux-64).

Changes

• recipe/recipe.yaml — moved stdlib('c') before the compiler conditional; updated variant description
• recipe/conda_build_config.yaml — gnu → gnu_omp; added per-variant c_stdlib_version with zip_keys
• .ci_support/ — updated runner_variant and c_stdlib_version in affected configs

Verification

All three Linux variants rebuilt and tested successfully with rattler-build:

Variant	Build string	__glibc	Tests
gnu_omp	gnu_omp_h6d0f835_1	>=2.17	✔
gnu_mpi	gnu_mpi_h1733be7_1	>=2.17	✔
intel	intel_h6facef5_1	>=2.28	✔