[gn build] find mistakes like the one fixed in 72b5989e0d1 at build time

diff --git a/llvm/utils/gn/build/write_cmake_config.py b/llvm/utils/gn/build/write_cmake_config.py
index d76d14b..ef70648 100755 (executable)
--- a/llvm/utils/gn/build/write_cmake_config.py
+++ b/llvm/utils/gn/build/write_cmake_config.py
@@ -73,6 +73,11 @@ def main():
         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)
             unused_values.discard(var)
         elif in_line.startswith('#cmakedefine '):