package/supertux: bump to version 0.6.0

package/supertux: bump to version 0.6.0

* Bump the minimum version of gcc since supertux require the C++14
  standard.
* Add boost locale dependency since it's now checked by the build
  system.
* Add libpng dependency for savepng feature.
* Add freetype dependency for the support for right-to-left languages.
* Disable ENABLE_OPENGLES2 for now, it can be enabled by a follow up
  patch.
* We don't add SDL_ttf dependency since it's included in the supertux
  source code.
* There is curently no libraqm package in Buildroot.
* Remove the two sed command from SUPERTUX_FIX_ALTIVEC_ISSUE since the
  CMake code has changed. Keep the one for tinygettext.

See:
https://www.supertux.org/news/2018/12/23/0.6.0

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>