Fixed bug 1829 - sdl2-config is not executable

leighmanthegreat@hotmail.com

I downloaded SDL2 from hg source.
I built to a ./build directory.
I downloaded SDL_image 2 hg and attempted to build.
When it cannot find a sdl2-config the SDL_image configure correctly suggests setting SDL_CONFIG variable.
Setting this the configure still fails with 'Permission denied' on the call to sdl2-config.
Setting execute permission solves the problem.

If possible, sdl2-config should have executable bit set when it is created.