Help/guide: use GNUInstallDirs in importing-exporting example

This allows the example to also show how to work on systems with
different libdir settings (e.g., Debian multiarch or Red Hat multilib)
rather than a regular `lib` directory.

Help/guide/importing-exporting/MathFunctions/CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required(VERSION 3.15)
 project(MathFunctions)
 
+# make cache variables for install destinations
+include(GNUInstallDirs)
+
 # specify the C++ standard
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
@@ -12,26 +15,26 @@ add_library(MathFunctions STATIC MathFunctions.cxx)
 target_include_directories(MathFunctions
                            PUBLIC
                            "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
-                           "$<INSTALL_INTERFACE:include>"
+                           "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
 )
 
 # install the target and create export-set
 install(TARGETS MathFunctions
         EXPORT MathFunctionsTargets
-        LIBRARY DESTINATION lib
-        ARCHIVE DESTINATION lib
-        RUNTIME DESTINATION bin
-        INCLUDES DESTINATION include
+        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+        INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
 )
 
 # install header file
-install(FILES MathFunctions.h DESTINATION include)
+install(FILES MathFunctions.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 # generate and install export file
 install(EXPORT MathFunctionsTargets
         FILE MathFunctionsTargets.cmake
         NAMESPACE MathFunctions::
-        DESTINATION lib/cmake/MathFunctions
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )
 
 # include CMakePackageConfigHelpers macro
@@ -58,14 +61,14 @@ write_basic_package_version_file(
 # create config file
 configure_package_config_file(${CMAKE_CURRENT_SOURCE_DIR}/Config.cmake.in
   "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfig.cmake"
-  INSTALL_DESTINATION lib/cmake/MathFunctions
+  INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )
 
 # install config files
 install(FILES
           "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfig.cmake"
           "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfigVersion.cmake"
-        DESTINATION lib/cmake/MathFunctions
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )
 
 # generate the export targets for the build tree

Help/guide/importing-exporting/MathFunctionsComponents/Addition/CMakeLists.txt

@@ -7,24 +7,24 @@ add_library(MathFunctions::Addition ALIAS Addition)
 target_include_directories(Addition
                            PUBLIC
                            "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
-                           $<INSTALL_INTERFACE:include>
+                           $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
 # install the target and create export-set
 install(TARGETS Addition
         EXPORT AdditionTargets
-        LIBRARY DESTINATION lib
-        ARCHIVE DESTINATION lib
-        RUNTIME DESTINATION bin
-        INCLUDES DESTINATION include
+        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+        INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
 )
 
 # install header file
-install(FILES Addition.h DESTINATION include)
+install(FILES Addition.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 # generate and install export file
 install(EXPORT AdditionTargets
         FILE MathFunctionsAdditionTargets.cmake
         NAMESPACE MathFunctions::
-        DESTINATION lib/cmake/MathFunctions
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )

Help/guide/importing-exporting/MathFunctionsComponents/CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required(VERSION 3.15)
 project(MathFunctionsComponents)
 
+# make cache variables for install destinations
+include(GNUInstallDirs)
+
 # specify the C++ standard
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
@@ -24,7 +27,7 @@ write_basic_package_version_file(
 # create config file
 configure_package_config_file(${CMAKE_CURRENT_SOURCE_DIR}/Config.cmake.in
   "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfig.cmake"
-  INSTALL_DESTINATION lib/cmake/MathFunctions
+  INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
   NO_CHECK_REQUIRED_COMPONENTS_MACRO
 )
 
@@ -32,5 +35,5 @@ configure_package_config_file(${CMAKE_CURRENT_SOURCE_DIR}/Config.cmake.in
 install(FILES
           "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfig.cmake"
           "${CMAKE_CURRENT_BINARY_DIR}/MathFunctionsConfigVersion.cmake"
-        DESTINATION lib/cmake/MathFunctions
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )

Help/guide/importing-exporting/MathFunctionsComponents/SquareRoot/CMakeLists.txt

@@ -7,24 +7,24 @@ add_library(MathFunctions::SquareRoot ALIAS SquareRoot)
 target_include_directories(SquareRoot
                            PUBLIC
                            "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
-                           "$<INSTALL_INTERFACE:include>"
+                           "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
 )
 
 # install the target and create export-set
 install(TARGETS SquareRoot
         EXPORT SquareRootTargets
-        LIBRARY DESTINATION lib
-        ARCHIVE DESTINATION lib
-        RUNTIME DESTINATION bin
-        INCLUDES DESTINATION include
+        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+        INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
 )
 
 # install header file
-install(FILES SquareRoot.h DESTINATION include)
+install(FILES SquareRoot.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 # generate and install export file
 install(EXPORT SquareRootTargets
         FILE MathFunctionsSquareRootTargets.cmake
         NAMESPACE MathFunctions::
-        DESTINATION lib/cmake/MathFunctions
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/MathFunctions
 )

Help/guide/importing-exporting/index.rst

@@ -202,6 +202,10 @@ project. Start by specifying the :command:`cmake_minimum_required` version and
   :language: cmake
   :end-before: # create library
 
+The :module:`GNUInstallDirs` module is included in order to provide the
+project with the flexibility to install into different platform layouts by
+making the directories available as cache variables.
+
 Create a library called ``MathFunctions`` with the :command:`add_library`
 command: