Make PACKET_DLL_DIR a cache variable on Windows.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index df80ccd..3363c5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,6 +12,9 @@
     option (USE_STATIC_RT "Use static Runtime" ON)
 endif( MSVC )
 option (BUILD_SHARED_LIBS "Build shared libraries" ON)
+if( WIN32 )
+    set(PACKET_DLL_DIR "" CACHE PATH "Path to directory with include and lib subdirectories for packet.dll")
+endif( WIN32 )
 
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 # Project setings