my/protobuf-c.m4: put the protoc-c check into a macro
my/protobuf-c.m4: put the protoc-c check into a macro

farsightsec/mtbl,hstern/nmsg,farsightsec/fstrm,hstern/nmsg,snarkmaster/mtbl,hstern/wdns,hstern/nmsg,snarkmaster/mtbl,rep/mtbl,hstern/wdns,rep/mtbl,hstern/nmsg,hstern/wdns,rep/mtbl,hstern/nmsg,farsightsec/fstrm,hstern/wdns,snarkmaster/mtbl,farsightsec/mtbl