Remove ::gets for FreeBSD 13 and later

Summary:
In https://svnweb.freebsd.org/changeset/base/351659 @emaste removed gets() from
FreeBSD 13's libc, and our copies of libc++ and libstdc++.  In that change, the
declarations were simply deleted, but I would like to propose this conditional
test instead.

Reviewers: EricWF, mclow.lists, emaste

Reviewed By: mclow.lists

Subscribers: krytarowski, christof, ldionne, emaste, libcxx-commits

Differential Revision: https://reviews.llvm.org/D67316

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@371324 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -1152,6 +1152,13 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #define _LIBCPP_HAS_NO_STDOUT
 #endif
 
+// Some systems do not provide gets() in their C library, for security reasons.
+#ifndef _LIBCPP_C_HAS_NO_GETS
+#  if defined(_LIBCPP_MSVCRT) || (defined(__FreeBSD__) && __FreeBSD__ >= 13)
+#    define _LIBCPP_C_HAS_NO_GETS
+#  endif
+#endif
+
 #if defined(__BIONIC__) || defined(__CloudABI__) ||                            \
     defined(__Fuchsia__) || defined(__wasi__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE

include/cstdio

@@ -152,7 +152,7 @@ using ::tmpnam;
 
 #ifndef _LIBCPP_HAS_NO_STDIN
 using ::getchar;
-#if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_MSVCRT)
+#if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_C_HAS_NO_GETS)
 using ::gets;
 #endif
 using ::scanf;