Add three download mirrors for 'bc', as the Makefile currently defines only
one download location.
@GNU can not be used, as the most recent version of 'bc' is not available at
the general GNU mirrors, and can only be found at the "gnu-alpha" mirrors.
Signed-off-by: Hannu Nyman <hannu.nyman at iki.fi>
Signed-off-by: Hauke Mehrtens <firstname.lastname@example.org>