Bundle grpc/protobuf in tiflash (#3896)

.gitmodules

@@ -65,3 +65,14 @@
 [submodule "contrib/benchmark"]
 	path = contrib/benchmark
 	url = https://github.com/google/benchmark
+[submodule "contrib/protobuf"]
+	path = contrib/protobuf
+	url = https://github.com/protocolbuffers/protobuf
+	branch = v3.8.0
+[submodule "contrib/abseil-cpp"]
+	path = contrib/abseil-cpp
+	url = https://github.com/abseil/abseil-cpp
+[submodule "contrib/grpc"]
+	path = contrib/grpc
+	url = https://github.com/grpc/grpc
+ 	branch = v1.26.0

CMakeLists.txt

@@ -282,6 +282,11 @@ if (UNBUNDLED OR NOT (ARCH_LINUX OR APPLE) OR ARCH_32)
     option (NO_WERROR "Disable -Werror compiler option" ON)
 endif ()
 
+if (PREBUILT_LIBS_ROOT)
+    list(PREPEND CMAKE_SYSTEM_PREFIX_PATH ${PREBUILT_LIBS_ROOT})
+    message(STATUS "Add ${PREBUILT_LIBS_ROOT} to cmake search path for pre-built libraries")
+endif()
+
 message (STATUS "Building for: ${CMAKE_SYSTEM} ${CMAKE_SYSTEM_PROCESSOR} ${CMAKE_LIBRARY_ARCHITECTURE} ; USE_STATIC_LIBRARIES=${USE_STATIC_LIBRARIES} MAKE_STATIC_LIBRARIES=${MAKE_STATIC_LIBRARIES} UNBUNDLED=${UNBUNDLED}")
 
 include(GNUInstallDirs)
@@ -318,6 +323,7 @@ include (cmake/find_execinfo.cmake)
 include (cmake/find_readline_edit.cmake)
 include (cmake/find_re2.cmake)
 include (cmake/find_llvm.cmake)
+include (cmake/find_protobuf.cmake)
 include (cmake/find_grpc.cmake)
 include (cmake/find_kvproto.cmake)
 include (cmake/find_tipb.cmake)