sdl2_gfx: new package

sdl2_gfx: new package

Signed-off-by: Peter Thompson <peter.macleod.thompson@gmail.com>
[Thomas:
 - Rewrap Config.in help text.
 - Remove unused SDL2_GFX_VERSION_MAJOR variable
 - Add host-pkgconf to the dependencies, since pkg-config is used to
   find sdl2. This fixes the build, otherwise -I/usr/include/sdl2 was
   added to the CFLAGS when sdl2-config was found on the host machine.
 - Remove --with-sdl2-prefix, since this option doesn't exist, and
   using pkg-config is a better solution (--with-sdl-prefix exists,
   but is not useful when pkg-config is available).
 - Remove --enable-static, packages are not supposed to pass such an
   option, it's not automatically by the autotools package
   infrastructure.
 - Pass --disable-sdltest instead of --disable-sdl2test since the
   latter doesn't exist, while the former does.
 - Pass SDL2_GFX_AUTORECONF = YES since the configure/Makefile.in are
   not up-to-date and therefore it tries to use aclocal at build time.
 - Adjust _LICENSE_FILES, since there is no file named 'LICENSE', use
   the smallest source file instead, since it contains the license
   text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>