FindProtobuf: Make outputs depend on protoc executable

After updating from Protocol Buffers 2.5.0 to 2.6.0 compilation of the
generated source failed: "This file was generated by an older version of
protoc which is incompatible with your Protocol Buffer headers. Please
regenerate this file with a newer version of protoc.".

Turns out the source and headers generated by way of
FindProtobuf.cmake:PROTOBUF_GENERATE_CPP aren't updated. Adding a
dependency on the compiler executable fixes this issue.

Modules/FindProtobuf.cmake

@@ -183,7 +183,7 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
              "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.h"
       COMMAND  ${PROTOBUF_PROTOC_EXECUTABLE}
       ARGS --cpp_out  ${CMAKE_CURRENT_BINARY_DIR} ${_protobuf_include_path} ${ABS_FIL}
-      DEPENDS ${ABS_FIL}
+      DEPENDS ${ABS_FIL} ${PROTOBUF_PROTOC_EXECUTABLE}
       COMMENT "Running C++ protocol buffer compiler on ${FIL}"
       VERBATIM )
   endforeach()