contrib: fix protobuf usage

Author: alalek

modules/cnn_3dobj/CMakeLists.txt

@@ -1,3 +1,5 @@
+set(BUILD_opencv_cnn_3dobj_INIT OFF)  # Must be disabled by default - requires custom build of Caffe.
+
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR})
 
 find_package(Caffe)
@@ -8,7 +10,9 @@ else()
   message(STATUS "Caffe:   NO")
 endif()
 
-find_package(Protobuf)
+if(NOT BUILD_PROTOBUF)
+  find_package(Protobuf)
+endif()
 if(Protobuf_FOUND)
   message(STATUS "Protobuf:   YES")
   set(HAVE_PROTOBUF 1)

modules/cnn_3dobj/FindProtobuf.cmake

@@ -79,7 +79,9 @@ add_definitions(-DCNN_NO_SERIALIZATION -DCNN_USE_CAFFE_CONVERTER)
 
 # NOTE: In case that proto files already exist,
 #       this is not needed anymore.
-find_package(Protobuf QUIET)
+if(NOT BUILD_PROTOBUF)
+  find_package(Protobuf QUIET)
+endif()
 
 if(NOT ${Protobuf_FOUND})
   message(STATUS "Module opencv_dnn_modern disabled because Protobuf is not found")