pkgconfig: add boost include headers path

[peckato1]

If nghttp2-asio was compiled with boost in a non-standard location, the generated pkg-config file did not include the boost include path, which could lead to compilation errors for projects depending on nghttp2-asio.

[gemini-code-assist]

Summary of Changes

Hello @peckato1, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a common build issue where projects linking against nghttp2-asio would encounter compilation errors due to missing Boost include paths in the generated pkg-config file. By integrating Boost's include directories into the pkg-config Cflags, this change ensures that dependent projects can correctly locate Boost headers, regardless of its installation location.

Highlights

• pkg-config Generation: The pkg-config file generation logic has been updated to correctly include Boost header paths, addressing issues where projects depending on nghttp2-asio would fail to compile if Boost was in a non-standard location.
• Build System Integration: A new CMake variable boostincludedir is introduced to capture the Boost_INCLUDE_DIRS and propagate it into the generated libnghttp2_asio.pc.in file's Cflags.

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[gemini-code-assist]

Code Review

This pull request aims to fix an issue where the Boost include path was not added to the pkg-config file for nghttp2-asio when Boost is installed in a non-standard location. While the intention is correct, the current implementation does not properly handle cases where Boost_INCLUDE_DIRS contains multiple paths, which would lead to incorrect Cflags in the generated pkg-config file. My review provides suggestions to create a more robust solution by iterating over all possible Boost include directories and formatting them correctly for the Cflags property, ensuring the fix works in all scenarios.