scons: Expose convienience libraries to the host environment.

This fixes MinGW cross compilation build, recently broken due to the use
of convenience libraries in the GLSL preprocessor.
diff --git a/SConstruct b/SConstruct
index 8e063e2..0a545d5 100644
--- a/SConstruct
+++ b/SConstruct
@@ -166,11 +166,14 @@
     host_env = Environment(
         # options are ignored
         # default tool is used
+        tools = ['default', 'custom'],
         toolpath = ['#scons'],	
         ENV = os.environ,
     )
 
     host_env['platform'] = common.default_platform
+    host_env['machine'] = common.default_machine
+    host_env['debug'] = env['debug']
 
     SConscript(
         'src/glsl/SConscript',