Make GSL includes based on CMAKE_INSTALL_INCLUDEDIR

This is to avoid inconsistencies with the `install`
command which specifies this as the destination for includes.

CMakeLists.txt

@@ -58,12 +58,12 @@ target_compile_definitions(GSL INTERFACE
 if(GSL_STANDALONE_PROJECT)
     target_include_directories(GSL INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-        $<INSTALL_INTERFACE:include>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
     )
 else()
     target_include_directories(GSL SYSTEM INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-        $<INSTALL_INTERFACE:include>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
     )
 endif()