download: improve handling of invalid local files

download: improve handling of invalid local files

4e19cbc5533: [download: handle possibly invalid local tarballs] added a
FORCE rule to downloaded files, so that they will be always checked by
download.pl.

As a side-effect, check-compile will fail, forcing unnecessary package
rebuilds.
The check-compile.txt log shows (for libxml2 for example):
  Considering target file '.../dl/libxml2-2.9.12.tar.gz'.
    ...
    prerequisite 'FORCE' of target '.../dl/libxml2-2.9.12.tar.gz' does
    not exist.
    Must remake target '.../dl/libxml2-2.9.12.tar.gz'.
    ...
   Giving up on target file '...libxml2-2.9.12/.prepared_...'.
   Giving up on target file '...libxml2-2.9.12/.configured_...'.
   Giving up on target file '...libxml2-2.9.12/.built'.
   Giving up on target file '...stamp/.libxml2_installed'.
  Giving up on target file '.compile'.

Then the package is rebuilt even if it is not otherwise needed.

To fix this, instead of always forcing the download target to be remade,
check its hash first: if it matches, then the FORCE is not added.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>