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

llvm-svn: 371324
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5bd4a4806aafc31c67a55c7d8b2993c879dc5bc2
diff --git a/include/cstdio b/include/cstdio
index 6755693..0f3f42d 100644
--- a/include/cstdio
+++ b/include/cstdio
@@ -152,7 +152,7 @@
 
 #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;