Index: llvm/utils/gn/build/write_cmake_config.py
===================================================================
--- llvm/utils/gn/build/write_cmake_config.py
+++ llvm/utils/gn/build/write_cmake_config.py
@@ -33,13 +33,13 @@
 """
 
 from __future__ import print_function
+from distutils.util import strtobool
 
 import argparse
 import os
 import re
 import sys
 
-
 def main():
     parser = argparse.ArgumentParser(
                  epilog=__doc__,
@@ -69,16 +69,16 @@
         def repl(m):
             key = m.group(1) or m.group(2)
             unused_values.discard(key)
-            return values[key]
+            return values.get(key)
         in_line = var_re.sub(repl, in_line)
         if in_line.startswith('#cmakedefine01 '):
             _, var = in_line.split()
-            if values[var] == '0':
-                print('error: "%s=0" used with #cmakedefine01 %s' % (var, var))
-                print("       '0' evaluates as truthy with #cmakedefine01")
-                print('       use "%s=" instead' % var)
-                return 1
-            in_line = '#define %s %d\n' % (var, 1 if values[var] else 0)
+            bool_value = values.get(var)
+            try:
+                bool_value = strtobool(bool_value)
+            except:
+                bool_value = None
+            in_line = '#define %s %d\n' % (var, 1 if bool_value else 0)
             unused_values.discard(var)
         elif in_line.startswith('#cmakedefine '):
             _, var = in_line.split(None, 1)
@@ -88,7 +88,7 @@
             except:
                 var = var.rstrip()
                 in_line = '#define %s\n' % var
-            if not values[var]:
+            if not values.get(var):
                 in_line = '/* #undef %s */\n' % var
             unused_values.discard(var)
         out_lines.append(in_line)