Change default hashing algorithm in file digests to SHA-256 (#485826)

- as per https://fedoraproject.org/wiki/Features/StrongerHashes
- patch from  Miloslav Trmač

macros

@@ -158,6 +158,10 @@ unset DISPLAY\
 # Should unpackaged files in a build root terminate a build?
 %_unpackaged_files_terminate_build    1
 
+# Use SHA-256 for FILEDIGESTS instead of default MD5
+%_source_filedigest_algorithm 8
+%_binary_filedigest_algorithm 8
+
 %__find_provides        /usr/lib/rpm/redhat/find-provides
 %__find_requires        /usr/lib/rpm/redhat/find-requires