cmake: copy include/libwebsockets into the build dir

https://libwebsockets.org/pipermail/libwebsockets/2018-November/007745.html

This allows user code to use the build dir ./include without
having to do the make install, as a convenience.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a904d86..820e4f8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1278,6 +1278,13 @@
 		      COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/include/libwebsockets.h
 		     					 ${CMAKE_CURRENT_BINARY_DIR}/include/libwebsockets.h
 	)
+
+	add_custom_command(
+		      TARGET websockets
+		      COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/include/libwebsockets/
+									${CMAKE_CURRENT_BINARY_DIR}/include/libwebsockets
+	)
+
 	add_custom_command(
 		      TARGET websockets
 		      COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_BINARY_DIR}/lws_config.h
@@ -1318,6 +1325,13 @@
 		      COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/include/libwebsockets.h
 		     					 ${CMAKE_CURRENT_BINARY_DIR}/include/libwebsockets.h
 	)
+
+	add_custom_command(
+		      TARGET websockets
+		      COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/include/libwebsockets
+									${CMAKE_CURRENT_BINARY_DIR}/include/libwebsockets
+	)
+
 	add_custom_command(
 		      TARGET websockets_shared
 		      COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_BINARY_DIR}/lws_config.h