[libc++] Exclude posix_l/strtonum fallback inclusion for newlib > 2.4

Summary:
[libc++] Exclude posix_l/strtonum fallback inclusion for newlib > 2.4

r338122 changed the linkage of some methods which revealed an existing ODR violation, e.g.:
projects/libcxx/include/support/xlocale/__posix_l_fallback.h:83:38: error: 'internal_linkage' attribute does not appear on the first declaration of 'iswcntrl_l'
inline _LIBCPP_INLINE_VISIBILITY int iswcntrl_l(wint_t c, locale_t) {
                                     ^
lib/include/wctype.h:55:12: note: previous definition is here
extern int      iswcntrl_l (wint_t, locale_t);

These were added to newlib in 2.4 [1] [2], so move them to the already existing include guard.

[1] https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=commit;h=238455adfab4f8070ac65400aac22bb8a9e502fc
[2] https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=commit;h=8493c1631643fada62384768408852bc0fa6ff44

Reviewers: ldionne, rsmith, EricWF

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D49927

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338157 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed