cmake: use relative path CMAKE_* variables

CMakeLists.txt

@@ -189,7 +189,7 @@ target_compile_definitions(drjit-core PRIVATE -DLZ4LIB_VISIBILITY=)
 target_include_directories(drjit-core
   PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-  $<INSTALL_INTERFACE:include>)
+  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
 target_compile_definitions(drjit-core PRIVATE -DDRJIT_BUILD=1)
 
@@ -250,10 +250,10 @@ if (DRJIT_ENABLE_TESTS)
 endif()
 
 configure_file(drjit-core-config.cmake "${CMAKE_CURRENT_BINARY_DIR}/drjit-core/drjit-core-config.cmake")
-install(TARGETS drjit-core EXPORT drjit-core-targets INCLUDES DESTINATION include)
-install(DIRECTORY include/drjit-core DESTINATION include)
-install(FILES drjit-core-config.cmake DESTINATION lib/cmake/drjit-core)
+install(TARGETS drjit-core EXPORT drjit-core-targets INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+install(DIRECTORY include/drjit-core DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+install(FILES drjit-core-config.cmake DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/drjit-core)
 install(
   EXPORT drjit-core-targets
   FILE drjit-core-targets.cmake
-  DESTINATION lib/cmake/drjit-core)
+  DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/drjit-core)