Fixed handling of the SCons construction variable "FRAMEWORKS"

This variable is only needed on OS X, but for simplicity, we expect it
to be defined (but empty) on other platforms.

SConstruct

@@ -148,6 +148,9 @@ if os.name == 'nt':
     for key,val in env['ENV'].iteritems():
         env['ENV'][key] = str(val)
 
+if 'FRAMEWORKS' not in env:
+    env['FRAMEWORKS'] = []
+
 add_RegressionTest(env)
 
 class defaults: pass