bpo-28503: Use crypt_r() when available instead of crypt() (GH-11373) (GH-11376)

Use crypt_r() when available instead of crypt() in the crypt module.

As a nice side effect: This also avoids a memory sanitizer flake as clang msan doesn't know about crypt's internal libc allocated buffer.
(cherry picked from commit 387512c7ecde6446f2e29408af2e16b9fc043807)

Co-authored-by: Gregory P. Smith <greg@krypto.org> [Google]
diff --git a/configure.ac b/configure.ac
index a7de901..788c361 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3875,6 +3875,23 @@
     ])
 )
 
+# We search for both crypt and crypt_r as one or the other may be defined
+# This gets us our -lcrypt in LIBS when required on the target platform.
+AC_SEARCH_LIBS(crypt, crypt)
+AC_SEARCH_LIBS(crypt_r, crypt)
+
+AC_CHECK_FUNC(crypt_r,
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#define _GNU_SOURCE  /* Required for crypt_r()'s prototype in glibc. */
+#include <crypt.h>
+]], [[
+struct crypt_data d;
+char *r = crypt_r("", "", &d);
+]])],
+    [AC_DEFINE(HAVE_CRYPT_R, 1, [Define if you have the crypt_r() function.])],
+    [])
+)
+
 AC_CHECK_FUNCS(clock_gettime, [], [
     AC_CHECK_LIB(rt, clock_gettime, [
         LIBS="$LIBS -lrt"