Allow users to specify a scons build cache directory.

SConstruct

Index: SConstruct
===================================================================
--- SConstruct	(revision 4842)
+++ SConstruct	(working copy)
@@ -782,6 +782,7 @@
   result = Options()
   result.Add('mode', 'compilation mode (debug, release)', 'release')
   result.Add('sample', 'build sample (shell, process, lineprocessor)', '')
+  result.Add('cache', 'directory to use for scons build cache', '')
   result.Add('env', 'override environment settings (NAME0:value0,NAME1:value1,...)', '')
   result.Add('importenv', 'import environment settings (NAME0,NAME1,...)', '')
   for (name, option) in SIMPLE_OPTIONS.iteritems():
@@ -863,6 +864,8 @@
     Abort("Shared Object soname not applicable for static library.")
   if env['os'] != 'win32' and env['pgo'] != 'off':
     Abort("Profile guided optimization only supported on Windows.")
+  if env['cache'] and not os.path.isdir(env['cache']):
+    Abort("The specified cache directory does not exist.")
   if not (env['arch'] == 'arm' or env['simulator'] == 'arm') and ('unalignedaccesses' in ARGUMENTS):
     print env['arch']
     print env['simulator']
@@ -1131,6 +1134,8 @@
   else:
     env.Default('library')
 
+  if env['cache']:
+    CacheDir(env['cache'])
 
 # We disable deprecation warnings because we need to be able to use
 # env.Copy without getting warnings for compatibility with older