Automated attempt by @leftibot. Opening as draft — verification failed.

Reason: no test files were detected in the diff

What changed

Fix #631: Respect CMAKE_INSTALL_PREFIX and use GNUInstallDirs for install paths
The CMakeLists.txt used CMAKE_INSTALL_LIBDIR in install destinations without
including the GNUInstallDirs module, leaving the variable empty. This caused
libraries to install to absolute paths like /chaiscript instead of lib/chaiscript,
and the pkg-config file to install to /pkgconfig instead of lib/pkgconfig. The
chaiscript.pc.in template also hardcoded "lib" and "include" instead of using
the CMake variables. Added include(GNUInstallDirs) and updated the .pc.in
template to use @CMAKE_INSTALL_LIBDIR@ and @CMAKE_INSTALL_INCLUDEDIR@.
Co-Authored-By: Claude Opus 4.6 (1M context) [email protected]

Files

 CMakeLists.txt                     |  9 ++++++++-
 cmake/check_pkgconfig.cmake        | 32 ++++++++++++++++++++++++++++++++
 contrib/pkgconfig/chaiscript.pc.in |  4 ++--
 3 files changed, 42 insertions(+), 3 deletions(-)

Will close #631 once finalized.

Triggered by @lefticus.