SDL_gfx: propagate SDL to -dev outputs

`SDL_gfx.pc` refers to `sdl.pc`. In addition to that some `SDL_gfx`
headers import `SDL.h` directly. I noticed it when attempted to build a
small SDL demo project which imports only `SDL_gfx`. It out to have
worked. Instead build fails as:

    $ SDL1_CFLAGS=$(shell pkg-config SDL_gfx --cflags)
    ...
    Package sdl was not found in the pkg-config search path.
    Perhaps you should add the directory containing `sdl.pc'
    to the PKG_CONFIG_PATH environment variable

This change is enough to make it work.

0 files changed, 0 insertions, 0 deletions