scons: Provide shorthand aliases for software winsyses.

src/gallium/winsys/sw/gdi/SConscript

@@ -20,4 +20,5 @@ if env['platform'] == 'windows':
            'gdi_sw_winsys.c',
         ]
     )
+    env.Alias('ws_gdi', ws_gdi)
     Export('ws_gdi')

src/gallium/winsys/sw/null/SConscript

@@ -18,4 +18,5 @@ ws_null = env.ConvenienceLibrary(
        'null_sw_winsys.c',
     ]
 )
+env.Alias('ws_null', ws_null)
 Export('ws_null')

src/gallium/winsys/sw/xlib/SConscript

@@ -20,4 +20,5 @@ if env['platform'] in ('cygwin', 'darwin', 'freebsd', 'linux', 'sunos'):
            'xlib_sw_winsys.c',
         ]
     )
+    env.Alias('ws_xlib', ws_xlib)
     Export('ws_xlib')