use target_include_directories instead of include_directories

CMakeLists.txt

@@ -35,8 +35,6 @@ set( HEADER_FOLDER "include" )
 set( SOURCE_FOLDER "src" )
 set( TEST_FOLDER "test" )
 
-include_directories( ${HEADER_FOLDER} )
-
 # This is needed so IDE's live MSVC show header files
 set( HEADER_FILES
 	${HEADER_FOLDER}/date/chrono_io.h
@@ -57,6 +55,16 @@ endif( )
 set_property(TARGET tz PROPERTY CXX_STANDARD 14)
 target_link_libraries( tz ${CMAKE_THREAD_LIBS_INIT} ${OPTIONAL_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT}  )
 
+# add include folders to the library and targets that consume it
+target_include_directories(tz PUBLIC
+    $<BUILD_INTERFACE:
+        ${CMAKE_CURRENT_SOURCE_DIR}/${HEADER_FOLDER}
+    >
+    $<INSTALL_INTERFACE:
+        include
+    >
+)
+
 install( TARGETS tz DESTINATION lib )
 install( DIRECTORY ${HEADER_FOLDER}/ DESTINATION include/ )