disable opencv dependency

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -190,7 +190,7 @@ if (WITH_LUAJIT)
     add_definitions(-DHAVE_LUAJIT=1)
 else()
     message(STATUS "Building with Lua (but not LuaJIT) support")
-    find_package(Lua REQUIRED)
+    find_package(Lua ${MODLUA_VERSION} REQUIRED)
     include_directories(SYSTEM ${LUA_INCLUDE_DIR})
 endif()
 
@@ -211,6 +211,7 @@ find_path(POTRACE_INCLUDE_DIR potracelib.h)
 find_library(POTRACE_LIBRARY NAMES potrace)
 
 find_package(OpenCV QUIET OPTIONAL_COMPONENTS core imgcodecs imgproc)
+set(OPENCV_CORE_FOUND 0)
 
 ############### Libraries are found now ########################
 
