contrib: protoc: build without the host name

protoc generates platform-independent source code. We don't need to have one version per target OS.

As explained in 104ac390, we still need this over just picking the right tool with AC_CHECK_TOOL(). Otherwise the PROTOC variable doesn't have the PATH in it and we would need to keep the environment PATH the same as it call to 'make'.

If protoc doesn't come from the contribs, we assume it's always in the PATH.