scons: Revamp how to specify targets to build.

Use scons target and dependency system instead of ad-hoc options.

Now is simply a matter of naming what to build. For example:

  scons libgl-xlib

  scons libgl-gdi

  scons graw-progs

  scons llvmpipe

and so on. And there is also the possibility of scepcified subdirs, e.g.

  scons src/gallium/drivers

If nothing is specified then everything will be build.

There might be some rough corners over the next days. Please bare with me.

src/SConscript

@@ -1,19 +1,17 @@
 Import('*')
 
-if 'egl' in env['statetrackers']:
-    SConscript('mapi/vgapi/SConscript')
+SConscript('mapi/vgapi/SConscript')
+
+if env['platform'] == 'windows':
     SConscript('egl/main/SConscript')
+    SConscript('talloc/SConscript')
 
-if 'mesa' in env['statetrackers']:
-    if platform == 'windows':
-        SConscript('talloc/SConscript')
+SConscript('glsl/SConscript')
+SConscript('mapi/glapi/SConscript')
+SConscript('mesa/SConscript')
 
-    SConscript('glsl/SConscript')
-    SConscript('mapi/glapi/SConscript')
-    SConscript('mesa/SConscript')
-
-    if platform != 'embedded':
-        SConscript('glut/glx/SConscript')
+if env['platform'] != 'embedded':
+    SConscript('glut/glx/SConscript')
 
 SConscript('gallium/SConscript')