nng-sys: statically specifiy the lib output directory

[flxo]

CMake's GNUInstallDirs module sets CMAKE_INSTALL_LIBDIR to lib64 on some 64-bit Linux distributions (e.g., Ubuntu with multilib), but the build script only searches in lib, causing "could not find native static library nng" errors.

Explicitly set CMAKE_INSTALL_LIBDIR to lib to ensure consistent library output path across all platforms.

Fixes #38

Summary by CodeRabbit

• Bug Fixes
  • Fixed library installation directory configuration during build process to ensure consistent placement across different Linux distributions.

[coderabbitai]

📝 Walkthrough

Walkthrough

The nng-sys build script is modified to explicitly set CMAKE_INSTALL_LIBDIR to "lib" during the CMake configuration, ensuring the vendored NNG library is installed to a consistent directory path regardless of platform-specific defaults that may use lib64 on certain Linux distributions.

Changes

Cohort / File(s)	Summary
CMake Build Configuration nng-sys/build.rs	Adds CMAKE_INSTALL_LIBDIR variable definition to force library installation to "lib" directory, resolving platform-specific lib64 installation path issues on Linux systems.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 On Linux where lib64 once roamed,
A simple "lib" brings the library home,
One variable set, the build now stays true,
Ubuntu builds pass—problem through and through! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title directly describes the main change: statically specifying the lib output directory in nng-sys to resolve the CMAKE_INSTALL_LIBDIR inconsistency across platforms.
Linked Issues check	✅ Passed	The pull request addresses issue #38 by setting CMAKE_INSTALL_LIBDIR to lib, ensuring the static library is placed consistently in lib across platforms, matching the build script's expectations.
Out of Scope Changes check	✅ Passed	The change is narrowly scoped to setting CMAKE_INSTALL_LIBDIR in the CMake configuration for the vendored NNG build, directly addressing the library output directory issue without introducing unrelated modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[flxo]

This PR also need to be applied to the v1.xx branch.

[jonhoo]

Feels potentially more reasonable to me to update the build.rs to search in both lib64/ and lib/ (probably in that order), no?

[flxo]

Feels potentially more reasonable to me to update the build.rs to search in both lib64/ and lib/ (probably in that order), no?

Ok - fine for me. Your proposed change is in #29. Closing this in favor if #29