cmake: Use GENERATE_EXTENSIONS parameter in protobuf_generate

cmake/protobuf-config.cmake.in

@@ -38,11 +38,11 @@ function(protobuf_generate)
     set(_dll_export_decl "dllexport_decl=${protobuf_generate_EXPORT_MACRO}:")
   endif()
 
-  if(NOT protobuf_GENERATE_EXTENSIONS)
+  if(NOT protobuf_generate_GENERATE_EXTENSIONS)
     if(protobuf_generate_LANGUAGE STREQUAL cpp)
-      set(protobuf_GENERATE_EXTENSIONS .pb.h .pb.cc)
+      set(protobuf_generate_GENERATE_EXTENSIONS .pb.h .pb.cc)
     elseif(protobuf_generate_LANGUAGE STREQUAL python)
-      set(protobuf_GENERATE_EXTENSIONS _pb2.py)
+      set(protobuf_generate_GENERATE_EXTENSIONS _pb2.py)
     else()
       message(SEND_ERROR "Error: protobuf_generate given unknown Language ${LANGUAGE}, please provide a value for GENERATE_EXTENSIONS")
       return()
@@ -91,7 +91,7 @@ function(protobuf_generate)
     get_filename_component(_basename ${_proto} NAME_WE)
 
     set(_generated_srcs)
-    foreach(_ext ${protobuf_GENERATE_EXTENSIONS})
+    foreach(_ext ${protobuf_generate_GENERATE_EXTENSIONS})
       list(APPEND _generated_srcs "${CMAKE_CURRENT_BINARY_DIR}/${_basename}${_ext}")
     endforeach()
     list(APPEND _generated_srcs_all ${_generated_srcs})