$< used in non-suffix rules

[efuss]

• Version of collectd: 5.8.0
• Operating system / distribution: NetBSD
• Kernel version (if applicable):

Expected behavior

Building grpc working with non-GNU make

Actual behavior

Building grpc plugin fails with protoc being called with no input files.

Steps to reproduce

Build grpc plugin with bmake.

Makefile.ac, for the collectd.grpc.pb.cc and collectd.pb.cc targets, uses $< (implied source) in the rule calling protoc, which is only valid for suffix (inference) rules. That should be $> (all sources) or spelled out.

[rubenk]

If you could send a PR which doesn't break building with Gnu Make I'd be willing to accept it.

[efuss]

#3330. I hope I didn't mix anything up (I'm more familiar with sending patches than Pull Requests).