[libcxx] [regex] Use distinct __regex_word on NetBSD

[mgorny]

Summary

NetBSD defines character classes up to 0x2000. Use 0x8000 as a safe
__regex_word that hopefully will not collide with other values
in the foreseeable future.

[krytarowski]

Where and what defines 0x2000, what header and what symbol name?

[krytarowski]

OK, I got it:

#define _CTYPE_Q 0x2000 /* Phonogram */

from /usr/include/sys/ctype_bits.h

I propose to mention the symbol name in the description and the header. In the description is worth to mention the filename of this definition.

include/regex

 public:
     typedef _CharT                  char_type;
     typedef basic_string<char_type> string_type;
     typedef locale                  locale_type;
     typedef ctype_base::mask        char_class_type;
 
 #if defined(__mips__) && defined(__GLIBC__)
     static const char_class_type __regex_word = static_cast<char_class_type>(_ISbit(15));
+#elif defined(__NetBSD__)
+    // NetBSD defines classes up to 0x2000
+    // see sys/ctype_bits.h, _CTYPE_Q
+    static const char_class_type __regex_word = 0x8000;
 #else
     static const char_class_type __regex_word = 0x80;
 #endif
 
 private:
     locale __loc_;
     const ctype<char_type>* __ct_;
     const collate<char_type>* __col_;