$NetBSD$

Do not build included libwebp, use system one.

--- CMakeLists.txt.orig	2019-09-24 03:40:00.000000000 +0000
+++ CMakeLists.txt
@@ -223,8 +223,8 @@ add_definitions(-DPNG_NO_MMX_CODE) # Do 
 # libwebp
 if(WITH_WEBP_SUPPORT)
   set(WEBP_LIBRARIES webp webpdemux webpmux)
-  set(WEBP_INCLUDE_DIR ${LIBWEBP_DIR}/src)
-  include_directories(${WEBP_INCLUDE_DIR})
+#  set(WEBP_INCLUDE_DIR ${LIBWEBP_DIR}/src)
+#  include_directories(${WEBP_INCLUDE_DIR})
 endif()
 
 # tinyxml