disable opencv dependency

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -188,7 +188,7 @@ if (WITH_LUA)
         add_definitions(-DHAVE_LUAJIT=1)
     else()
         message(STATUS "Building with Lua support")
-        find_package(Lua REQUIRED)
+        find_package(Lua ${MODLUA_VERSION} REQUIRED)
         include_directories(SYSTEM ${LUA_INCLUDE_DIR})
     endif()
     add_definitions(-DHAVE_LUA=1)
@@ -212,6 +212,7 @@ 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 ########################
 
