Anything necessary to prepare the spell to be built should be done here, like patching or unpacking additional sources. Usually this is done by adding commands after
default_pre_build, which unpacks the first source automatically:
default_pre_build && cd "$SOURCE_DIRECTORY" && unpack_file 2 && patch -p0 < "$SPELL_DIRECTORY/gcc4.patch"
Another use for this file is for spells using badly packaged sources that don't unpack into a directory:
mk_source_dir "$SOURCE_DIRECTORY" && cd "$SOURCE_DIRECTORY" && unpack_file