Patched to compile on C89 compilers.

src/video/mir/SDL_mirframebuffer.c

@@ -91,11 +91,13 @@ MIR_UpdateWindowFramebuffer(_THIS, SDL_Window* window,
     MirGraphicsRegion region;
     int i, j, x, y, w, h, start;
     int bytes_per_pixel, bytes_per_row, s_stride, d_stride;
+    char* s_dest;
+    char* pixels;
 
     MIR_mir_surface_get_graphics_region(mir_window->surface, &region);
 
-    char* s_dest = region.vaddr;
-    char* pixels = (char*)window->surface->pixels;
+    s_dest = region.vaddr;
+    pixels = (char*)window->surface->pixels;
 
     s_stride = window->surface->pitch;
     d_stride = region.stride;