package/gettext-gnu: bump version to 0.20.1

Other changes:
  - Update license hash due to http -> https URL changes.
  - Depend on host-libtextstyle for host-gettext-gnu.
  - Update 0001-error_print_progname.patch to apply cleanly.
  - Remove upstream 0002-Update-after-gnulib-changed.patch
  - Add 0002-restore-the-ability-to-buld-gettext-tools-seperately-part1.patch
    which allows gettext-tools to build with an external libtextstyle.

Tested under Debian 8 with test-pkg -a:
44 builds, 5 skipped, 0 build failed, 0 legal-info failed

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>