Respect CMAKE_INSTALL_LIBDIR, close #22

CMakeLists.txt

@@ -94,6 +94,6 @@ if (TARGET aptdec)
     install(DIRECTORY "${PROJECT_SOURCE_DIR}/palettes" DESTINATION ${CMAKE_INSTALL_DATADIR}/${CMAKE_PROJECT_NAME})
 endif()
 
-install(TARGETS apt PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/apt LIBRARY DESTINATION lib)
+install(TARGETS apt PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/apt LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})
 
 include(CPack)