don't dpkg on windows and macos

No need to clutter windows and macos configuration with dpkg options, as it is
unlikely to ever be useful.

CMakeLists.txt

@@ -50,10 +50,12 @@ option(USE_MPG123 "use mpg123 + libsndfile for sound" ON)
 # OS X deployment
 option(OPENMW_OSX_DEPLOYMENT OFF)
 
-option(BUILD_WITH_DPKG "enable dpkg-based install for debian and debian derivatives" OFF)
-if(BUILD_WITH_DPKG)
-    find_program(DPKG_PROGRAM dpkg DOC "dpkg program of Debian-based systems")
-endif(BUILD_WITH_DPKG)
+if(UNIX AND NOT APPLE)
+    option(BUILD_WITH_DPKG "enable dpkg-based install for debian and debian derivatives" OFF)
+    if(BUILD_WITH_DPKG)
+        find_program(DPKG_PROGRAM dpkg DOC "dpkg program of Debian-based systems")
+    endif(BUILD_WITH_DPKG)
+endif(UNIX AND NOT APPLE)
 
 # Location of morrowind data files
 if (APPLE)