libgl-gdi: support building without softpipe

While we do need *some* fallback-driver, there's no good reason to
*always* require that to be Softpipe. LLVMpipe for instance does the job
just fine.

This makes the minimal build a bit smaller on Windows.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7506>

src/gallium/targets/libgl-gdi/SConscript

@@ -29,6 +29,7 @@ sources = ['libgl_gdi.c']
 drivers = []
 
 if True:
+    env.Append(CPPDEFINES = ['GALLIUM_SOFTPIPE'])
     drivers += [softpipe]
 
 if env['llvm']:

src/gallium/targets/libgl-gdi/libgl_gdi.c

@@ -41,9 +41,11 @@
 #include "stw_device.h"
 #include "gdi/gdi_sw_winsys.h"
 
+#ifdef GALLIUM_SOFTPIPE
 #include "softpipe/sp_texture.h"
 #include "softpipe/sp_screen.h"
 #include "softpipe/sp_public.h"
+#endif
 
 #ifdef GALLIUM_LLVMPIPE
 #include "llvmpipe/lp_texture.h"
@@ -78,8 +80,10 @@ gdi_screen_create(void)
    default_driver = "llvmpipe";
 #elif GALLIUM_SWR
    default_driver = "swr";
-#else
+#elif defined(GALLIUM_SOFTPIPE)
    default_driver = "softpipe";
+#else
+#error "no suitable default-driver"
 #endif
 
    driver = debug_get_option("GALLIUM_DRIVER", default_driver);
@@ -100,11 +104,11 @@ gdi_screen_create(void)
 #endif
    (void) driver;
 
-   if (screen == NULL) {
+#ifdef GALLIUM_SOFTPIPE
+   if (screen == NULL)
       screen = softpipe_create_screen( winsys );
-   }
-
-   if(!screen)
+#endif
+   if (!screen)
       goto no_screen;
 
    return screen;
@@ -150,9 +154,11 @@ gdi_present(struct pipe_screen *screen,
    }
 #endif
 
+#ifdef GALLIUM_SOFTPIPE
    winsys = softpipe_screen(screen)->winsys,
    dt = softpipe_resource(res)->dt,
    gdi_sw_display(winsys, dt, hDC);
+#endif
 }