cmake: use relative path CMAKE_* variables

CMakeLists.txt

@@ -65,7 +65,7 @@ endif()
 target_include_directories(nanothread
   PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-  $<INSTALL_INTERFACE:include>)
+  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
 target_compile_definitions(nanothread PRIVATE -DNANOTHREAD_BUILD=1)
 set_target_properties(nanothread PROPERTIES INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE)
@@ -76,31 +76,25 @@ if (NANOTHREAD_ENABLE_TESTS)
 endif()
 
 configure_file(nanothreadConfig.cmake "${CMAKE_CURRENT_BINARY_DIR}/nanothread/nanothreadConfig.cmake" COPYONLY)
-install(
-  FILES
-    nanothreadConfig.cmake
-  DESTINATION
-    lib/cmake/nanothread
-)
 install(
   TARGETS
     nanothread
   EXPORT
     nanothreadTargets
   INCLUDES DESTINATION
-    include
+    ${CMAKE_INSTALL_INCLUDEDIR}
 )
 install(
   DIRECTORY
     include/nanothread
   DESTINATION
-    include
+    ${CMAKE_INSTALL_INCLUDEDIR}
 )
 install(
   EXPORT
     nanothreadTargets
   FILE
     nanothreadTargets.cmake
   DESTINATION
-    lib/cmake/nanothread
+    ${CMAKE_INSTALL_LIBDIR}/cmake/nanothread
 )