Guard glibc-only FPU control for musl builds

[MEO265]

Motivation

• Prevent build failures on non-glibc Linux (e.g. musl) by disabling use of glibc-only FPU control symbols when glibc is not present.

Description

• Add __GLIBC__ guard to the #include <fpu_control.h> in src/open_wbo/solvers/glucose4.1/utils/System.h so the header is only included on glibc systems.
• Restrict the runtime FPU-control setup blocks in src/open_wbo/Main.cc, src/open_wbo/solvers/glucose4.1/parallel/Main.cc, and src/open_wbo/solvers/glucose4.1/simp/Main.cc to only compile when both __linux__ and __GLIBC__ are defined.
• The changes avoid referencing glibc-only symbols on platforms that do not provide them while preserving existing behavior on glibc-based systems.

Testing

• Ran R -q -e 'lintr::lint_package()' which completed with no lints found.
• Ran R CMD build . which produced Rwbo_0.1.2.tar.gz successfully.
• Ran R CMD check --no-manual Rwbo_0.1.2.tar.gz which completed with Status: OK.

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Codex Task