Use changable Cmake includedir instead of static "include"

The CMAKE_INSTALL_INCLUDEDIR path (from GNUInstallDirs) lets the end-user change the install path.

Author: sum01

CMakeLists.txt

@@ -41,7 +41,8 @@ target_compile_features(${PROJECT_NAME} INTERFACE
 
 target_include_directories(${PROJECT_NAME} INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
-    $<INSTALL_INTERFACE:include>)
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+
 
 target_link_libraries(${PROJECT_NAME} INTERFACE
 	# Always require threads