can not specify installation dir for debuginfo file

[shdluan]

I just want to specify a new installation dir for debuginfo/debugsource packge, but I cannot find how to config it.
If add set _prefix, it just applied for binary files, the debug rpm's installation path are still in /usr/lib/debug or /usr/src/debug.
I'd like to know how to config rpmbuild command to support customization installation path for debug rpms