Fix the __glibcxx_min and __glibcxx_max macros for a signed wchar_t.
Fix the __glibcxx_min and __glibcxx_max macros for a signed wchar_t.

* The __glibcxx_max macro came from GCC svn r138078, the last GPLv2
  revision of this file.
* I wrote the updated __glibcxx_min macro.

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