SConstruct: make help work even if guesses failed.

SConstruct

Index: SConstruct
diff --git a/SConstruct b/SConstruct
index e33d2df46905185dcb4592e2a17cb6ae53e34769..a36a96f9b31fbc284b45444610e4f01288000341 100644
--- a/SConstruct
+++ b/SConstruct
@@ -665,17 +665,17 @@ SIMPLE_OPTIONS = {
   'toolchain': {
     'values': ['gcc', 'msvc'],
     'default': TOOLCHAIN_GUESS,
-    'help': 'the toolchain to use (' + TOOLCHAIN_GUESS + ')'
+    'help': 'the toolchain to use (%s)' % TOOLCHAIN_GUESS
   },
   'os': {
     'values': ['freebsd', 'linux', 'macos', 'win32', 'android', 'openbsd', 'solaris'],
     'default': OS_GUESS,
-    'help': 'the os to build for (' + OS_GUESS + ')'
+    'help': 'the os to build for (%s)' % OS_GUESS
   },
   'arch': {
     'values':['arm', 'ia32', 'x64', 'mips'],
     'default': ARCH_GUESS,
-    'help': 'the architecture to build for (' + ARCH_GUESS + ')'
+    'help': 'the architecture to build for (%s)' % ARCH_GUESS
   },
   'regexp': {
     'values': ['native', 'interpreted'],