package/sdl_ttf: switch to github, update version

package/sdl_ttf: switch to github, update version

SDL's canonical source is now on GitHub. The git commit that matches
the Hg commit we used so far is b13aa509166e, but we switch to the
HEAD of the SDL-1.2 branch, which just has two additional commits
touching the autotools files.

Signed-off-by: Gleb Mazovetskiy <glex.spb@gmail.com>
[yann.morin.1998@free.fr:
 - explain why we don;t use the exactly-maching commit
  - use full-length commit hash
  - drop the comment: it did not point to the commit we used
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>