Mine

--- rpm-4.4.2.2/build/files.c.pix       2007-09-11 08:28:12.000000000 +0200
+++ rpm-4.4.2.2/build/files.c   2007-10-16 14:52:13.000000000 +0200
@@ -1988,7 +1988,7 @@ static int processPackageFiles(Spec spec
     memset(&fl, 0, sizeof(fl));
 
     /* XXX spec->buildRootURL == NULL, then xstrdup("") is returned */
-    fl.buildRootURL = rpmGenPath(spec->rootURL, spec->buildRootURL, NULL);
+    fl.buildRootURL = rpmGenPath(spec->rootURL, spec->buildRootURL ? spec->buildRootURL : "%{?buildroot}", NULL);
 
     if (hge(pkg->header, RPMTAG_DEFAULTPREFIX, NULL, (void **)&fl.prefix, NULL))
        fl.prefix = xstrdup(fl.prefix);
--- rpm-4.4.2.2/build/rpmfc.c.pix       2007-09-11 08:28:12.000000000 +0200
+++ rpm-4.4.2.2/build/rpmfc.c   2007-10-16 14:14:57.000000000 +0200
@@ -1576,7 +1576,15 @@ int rpmfcGenerateDepends(const Spec spec
     fc->skipProv = !pkg->autoProv;
     fc->skipReq = !pkg->autoReq;
     fc->tracked = 0;
-    fc->brlen = (spec->buildRootURL ? strlen(spec->buildRootURL) : 0);
+
+    {  const char * buildRootURL;
+       const char * buildRoot;
+       buildRootURL = rpmGenPath(spec->rootURL, "%{?buildroot}", NULL);
+       (void) urlPath(buildRootURL, &buildRoot);
+       if (buildRoot && !strcmp(buildRoot, "/")) buildRoot = NULL;
+       fc->brlen = (buildRoot ? strlen(buildRoot) : 0);
+       buildRootURL = _free(buildRootURL);
+    }
 
     /* Copy (and delete) manually generated dependencies to dictionary. */
     if (!fc->skipProv) {