Fix compile error when using cmake and -DWITH_OTLP=ON -DCMAKE_FIND_ROOT_PATH=<PROTOBUF INSTALL PREFIX DIR> with custom prebuilt protobuf. #424

Author: owent

cmake/opentelemetry-proto.cmake

@@ -132,6 +132,19 @@ add_library(
   ${METRICS_SERVICE_PB_CPP_FILE}
   ${METRICS_SERVICE_GRPC_PB_CPP_FILE})
 
+if(TARGET protobuf::libprotobuf)
+  target_link_libraries(opentelemetry_proto
+    PUBLIC protobuf::libprotobuf
+  )
+else() # cmake 3.8 or lower
+  target_include_directories(opentelemetry_proto
+    PUBLIC ${Protobuf_INCLUDE_DIRS}
+  )
+  target_include_directories(opentelemetry_proto
+    INTERFACE ${Protobuf_LIBRARIES}
+  )
+endif()
+
 if(BUILD_SHARED_LIBS)
   set_property(TARGET opentelemetry_proto PROPERTY POSITION_INDEPENDENT_CODE ON)
 endif()