[BUILD] Fix cross-compilation with protoc (open-telemetry#3186)

Author: seanchann

CMakeLists.txt

@@ -428,12 +428,16 @@ if(WITH_OTLP_GRPC
   endif()
   # Latest Protobuf imported targets and without legacy module support
   if(TARGET protobuf::protoc)
-    project_build_tools_get_imported_location(PROTOBUF_PROTOC_EXECUTABLE
-                                              protobuf::protoc)
-    # If protobuf::protoc is not a imported target, then we use the target
-    # directly for fallback
-    if(NOT PROTOBUF_PROTOC_EXECUTABLE)
-      set(PROTOBUF_PROTOC_EXECUTABLE protobuf::protoc)
+    if(CMAKE_CROSSCOMPILING AND Protobuf_PROTOC_EXECUTABLE)
+      set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
+    else()
+      project_build_tools_get_imported_location(PROTOBUF_PROTOC_EXECUTABLE
+                                                protobuf::protoc)
+      # If protobuf::protoc is not a imported target, then we use the target
+      # directly for fallback
+      if(NOT PROTOBUF_PROTOC_EXECUTABLE)
+        set(PROTOBUF_PROTOC_EXECUTABLE protobuf::protoc)
+      endif()
     endif()
   elseif(Protobuf_PROTOC_EXECUTABLE)
     # Some versions of FindProtobuf.cmake uses mixed case instead of uppercase