Enable common subexpression optimization in GCC.

Enable common subexpression optimization in GCC.

* lib/dirent.in.h (_GL_ATTRIBUTE_PURE): Define, like in string.in.h.
(alphasort): Declare with GCC attribute 'pure'.
* lib/stdlib.in.h (_GL_ATTRIBUTE_PURE): Define, like in string.in.h.
(atoll): Declare with GCC attribute 'pure'.
* lib/string.in.h (mbslen, mbsnlen, mbschr, mbsrchr, mbsstr,
mbscasecmp, mbsncasecmp, mbspcasecmp, mbscasestr, mbscspn, mbspbrk,
mbsspn, strverscmp): Declare with GCC attribute 'pure'.
* lib/wchar.in.h (_GL_ATTRIBUTE_PURE): Define, like in string.in.h.
(btowc, wctob, wcwidth, wmemchr, wmemcmp, wcslen, wcsnlen, wcscmp,
wcsncmp, wcscasecmp, wcsncasecmp, wcschr, wcsrchr, wcscspn, wcsspn,
wcspbrk, wcsstr, wcswidth): Declare with GCC attribute 'pure'.