Ensure empty buildroot for %install

- remove any existing buildroot contents and safely create a new one
- patch originally from OpenSUSE / Michael Schroeder, adopted to Fedora
  by Tom "spot" Callaway

macros

@@ -73,6 +73,17 @@
 #==============================================================================
 # ---- Build policy macros.
 #
+#
+#---------------------------------------------------------------------
+#	Expanded at beginning of %install scriptlet.
+#
+
+%__spec_install_pre %{___build_pre}\
+    [ "$RPM_BUILD_ROOT" != "/" ] && rm -rf "${RPM_BUILD_ROOT}"\
+    mkdir -p `dirname "$RPM_BUILD_ROOT"`\
+    mkdir "$RPM_BUILD_ROOT"\
+%{nil}
+
 #---------------------------------------------------------------------
 #	Expanded at end of %install scriptlet.
 #