[cmake] boost check supports in-tree targets

[evaleev]

currently int_checkboost assumed IMPORTED Boost::headers, but this may not be true if Boost is built as part of the source tree. This adds support for non-IMPORTED Boost::headers, and the modularized variant which must use Boost::preprocessor

[Copilot]

Pull request overview

This PR enhances CMake configuration to support Boost when built as part of the source tree, rather than assuming it's always an imported target. The changes address both monolithic and modularized Boost variants, and include a small bug fix in the Eigen3 variable check.

Changes:

• Added logic to detect and handle non-imported Boost targets by extracting include directories from generator expressions
• Fixed incorrect variable reference syntax for LIBINT_HAS_EIGEN check
• Extended Eigen3 version detection to support Eigen 5.x header locations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
export/CMakeLists.txt.export	Removed erroneous ${} syntax around LIBINT_HAS_EIGEN variable check
cmake/modules/int_checkboost.cmake	Added support for non-imported Boost targets by extracting include directories from BUILD_INTERFACE generator expressions
cmake/modules/FindEigen3.cmake	Added fallback logic to detect Eigen version headers in both 5.x and 3.x locations

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