[gn build] Use print_function in write_cmake_config.py

[thakis]

Summary

No behavior change, just makes the script match the other scripts in llvm/utils/gn/build.

[phosek]

LGTM

llvm/utils/gn/build/write_cmake_config.py

set:
 
         #define FOO asdfBAR_VALjkl
 
 Fails if any of the KEY=VALUE arguments aren't needed for processing the
 .h.cmake file, or if the .h.cmake file has unreplaces ${VAR} references after
 processing all values.
 """
 
+from __future__ import print_function
+
 import argparse
 import os
 import re
 import sys
 
 
 def main():
     parser = argparse.ArgumentParser(
for in_line in in_lines:
             else:
                 out_lines.append('/* #undef %s */\n' % var)
             unused_values.discard(var)
         else:
             # In particular, handles `#define FOO ${FOO}` lines.
             out_lines.append(var_re.sub(repl, in_line))
 
     if unused_values:
-        print >>sys.stderr, 'Unused --values args:'
-        print >>sys.stderr, '    ', '\n    '.join(unused_values)
+        print('Unused --values args:', file=sys.stderr)
+        print('    ', '\n    '.join(unused_values), file=sys.stderr)
         return 1
 
     output = ''.join(out_lines)
 
     leftovers = var_re.findall(output)
     if leftovers:
-        print >>sys.stderr, 'unprocessed values:\n', '\n'.join(leftovers)
+        print('unprocessed values:\n', '\n'.join(leftovers), file=sys.stderr)
         return 1
 
     if not os.path.exists(args.output) or open(args.output).read() != output:
         open(args.output, 'w').write(output)
 
 
 if __name__ == '__main__':
     sys.exit(main())