Support configurable include install dir

This is useful for split-output packaging, where the
development headers are separate from the runtime libraries.

CMakeLists.txt

@@ -16,8 +16,13 @@ endif()
 
 if (UNIX AND NOT APPLE)
     include(GNUInstallDirs)
-elseif(NOT DEFINED CMAKE_INSTALL_LIBDIR)
-    set(CMAKE_INSTALL_LIBDIR "lib")
+else()
+    if(NOT DEFINED CMAKE_INSTALL_INCLUDEDIR)
+        set(CMAKE_INSTALL_INCLUDEDIR "include")
+    endif()
+    if(NOT DEFINED CMAKE_INSTALL_LIBDIR)
+        set(CMAKE_INSTALL_LIBDIR "lib")
+    endif()
 endif()
 
 # This is required in order to append /lib/cmake to each element in CMAKE_PREFIX_PATH
@@ -86,18 +91,18 @@ set_target_properties(${PROJECT_NAME} PROPERTIES VERSION 1.0.0)
 
 target_include_directories(${PROJECT_NAME} PUBLIC
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-        $<INSTALL_INTERFACE:include>)
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
 aws_use_package(aws-c-http)
 
 target_link_libraries(${PROJECT_NAME} PUBLIC ${DEP_AWS_LIBS})
 aws_prepare_shared_lib_exports(${PROJECT_NAME})
 
-install(FILES ${AWS_MQTT_HEADERS} DESTINATION "include/aws/mqtt" COMPONENT Development)
-install(FILES ${AWS_MQTT5_HEADERS} DESTINATION "include/aws/mqtt/v5" COMPONENT Development)
-install(FILES ${AWS_MQTT_RR_HEADERS} DESTINATION "include/aws/mqtt/request-response" COMPONENT Development)
-install(FILES ${AWS_MQTT_TESTING_HEADERS} DESTINATION "include/aws/testing/mqtt" COMPONENT Development)
-install(FILES ${AWS_MQTT_PRIV_EXPOSED_HEADERS} DESTINATION "include/aws/mqtt/private" COMPONENT Development)
+install(FILES ${AWS_MQTT_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/aws/mqtt" COMPONENT Development)
+install(FILES ${AWS_MQTT5_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/aws/mqtt/v5" COMPONENT Development)
+install(FILES ${AWS_MQTT_RR_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/aws/mqtt/request-response" COMPONENT Development)
+install(FILES ${AWS_MQTT_TESTING_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/aws/testing/mqtt" COMPONENT Development)
+install(FILES ${AWS_MQTT_PRIV_EXPOSED_HEADERS} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/aws/mqtt/private" COMPONENT Development)
 
 if (BUILD_SHARED_LIBS)
     set (TARGET_DIR "shared")