[gn build] Fix cosmetic bug in write_cmake_config.py

Before, #cmakedefine FOO resulted in #define FOO  with a trailing space if FOO
was set to something truthy. Make it so that it's just #define FOO without a
trailing space.

No functional difference.

Differential Revision: https://reviews.llvm.org/D55172

llvm-svn: 348107