Add cmake option to compile using protobuf-lite

After this change specifying -DGRPC_USE_PROTO_LITE=On at the cmake
command line will cause gRPC to use the protobuf-lite runtime what
can result in a major size improvement.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1f647e0..8ecac86 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -72,6 +72,13 @@
 set(CMAKE_C_FLAGS   "${CMAKE_C_FLAGS}   -std=c11")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 
+if (GRPC_USE_PROTO_LITE)
+  set(PROTOBUF_LIBRARY_NAME "libprotobuf-lite")
+  add_definitions("-DGRPC_USE_PROTO_LITE")
+else()
+  set(PROTOBUF_LIBRARY_NAME "libprotobuf")
+endif()
+
   
 add_library(gpr
   src/core/lib/profiling/basic_timers.c
@@ -742,7 +749,7 @@
 
 target_link_libraries(grpc++
   ssl
-  libprotobuf
+  ${PROTOBUF_LIBRARY_NAME}
   grpc
 )
 
@@ -809,7 +816,7 @@
 )
 
 target_link_libraries(grpc++_unsecure
-  libprotobuf
+  ${PROTOBUF_LIBRARY_NAME}
   gpr
   grpc_unsecure
   grpc