scons: Fix cross-compilation.

Hairy stuff. Don't know how to do it better though.
diff --git a/src/SConscript b/src/SConscript
index 38137ee..201812c 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -3,6 +3,9 @@
 if env['platform'] == 'windows':
     SConscript('getopt/SConscript')
     SConscript('talloc/SConscript')
+else:
+    talloc = 'talloc'
+    Export('talloc')
 
 SConscript('glsl/SConscript')
 SConscript('mapi/glapi/SConscript')