message: fix incorrect installation paths
  they should be relative to the prefix, thus using CMAKE_INSTALL_*DIR is incorrect
diff: @@ -5,12 +5,12 @@ function(ginkgo_install_library name subdir)
  # install .so and .a files
  install(TARGETS "${name}"
  EXPORT Ginkgo
  - LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  - ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
  + LIBRARY DESTINATION lib
  + ARCHIVE DESTINATION lib
  )
  # copy header files
  install(DIRECTORY "${PROJECT_SOURCE_DIR}/${subdir}"
  - DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
  + DESTINATION include
  FILES_MATCHING PATTERN "*.hpp"
  )
  endfunction()
  @@ -19,14 +19,14 @@ function(ginkgo_default_includes name)
  # set include path depending on used interface
  target_include_directories("${name}"
  PUBLIC
  - $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
  + $<INSTALL_INTERFACE:include>
  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>
  )
  endfunction()
  function(ginkgo_install)
  # export targets
  - set(INSTALL_CONFIG_DIR "${CMAKE_INSTALL_LIBDIR}/cmake/Ginkgo")
  + set(INSTALL_CONFIG_DIR "lib/cmake/Ginkgo")
  install(EXPORT Ginkgo
  FILE GinkgoTargets.cmake
  NAMESPACE Ginkgo::
  