scons: Get GLSL code building correctly when cross compiling.

This is quite messy. GLSL code has to be built twice: one for the
host OS, another for the target OS.
diff --git a/src/SConscript b/src/SConscript
index f7fac33..6083fcb 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -1,8 +1,6 @@
 Import('*')
 
-SConscript('glsl/pp/SConscript')
-SConscript('glsl/cl/SConscript')
-SConscript('glsl/apps/SConscript')
+SConscript('glsl/SConscript')
 SConscript('gallium/SConscript')
 
 if 'mesa' in env['statetrackers']: