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) <noreply@anthropic.com>