Handle relative and absolute paths for pkg-config file

CMakeLists.txt

@@ -143,8 +143,17 @@ if (EXIV2_TEAM_PACKAGING)
   include(cmake/packaging.cmake)
 endif()
 
-join_paths(libdir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
-join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+# Handle both relative and absolute paths (e.g. NixOS) for a relocatable package
+if(IS_ABSOLUTE "${CMAKE_INSTALL_LIBDIR}")
+    set(libdir_for_pc_file "${CMAKE_INSTALL_LIBDIR}")
+else()
+    join_paths(libdir_for_pc_file "\${exec_prefix}" "${CMAKE_INSTALL_LIBDIR}")
+endif()
+if(IS_ABSOLUTE "${CMAKE_INSTALL_INCLUDEDIR}")
+    set(includedir_for_pc_file "${CMAKE_INSTALL_INCLUDEDIR}")
+else()
+    join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+endif()
 configure_file(cmake/exiv2.pc.in exiv2.pc @ONLY)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/exiv2.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)