MEDIUM: auth/threads: make use of crypt_r() on systems supporting it

On systems where crypt_r() is available, prefer it over a locked crypt().
This improves performance especially on very slow crypto algorithms.
diff --git a/Makefile b/Makefile
index 2d17984..a008df6 100644
--- a/Makefile
+++ b/Makefile
@@ -293,6 +293,7 @@
   USE_NETFILTER   = implicit
   USE_POLL        = implicit
   USE_TPROXY      = implicit
+  USE_CRYPT_H     = implicit
   USE_LIBCRYPT    = implicit
   USE_DL          = implicit
   USE_RT          = implicit
@@ -304,6 +305,7 @@
   USE_EPOLL       = implicit
   USE_MY_EPOLL    = implicit
   USE_TPROXY      = implicit
+  USE_CRYPT_H     = implicit
   USE_LIBCRYPT    = implicit
   USE_DL          = implicit
   USE_RT          = implicit
@@ -314,6 +316,7 @@
   USE_POLL        = implicit
   USE_EPOLL       = implicit
   USE_TPROXY      = implicit
+  USE_CRYPT_H     = implicit
   USE_LIBCRYPT    = implicit
   USE_FUTEX       = implicit
   USE_DL          = implicit
@@ -325,6 +328,7 @@
   USE_POLL        = implicit
   USE_EPOLL       = implicit
   USE_TPROXY      = implicit
+  USE_CRYPT_H     = implicit
   USE_LIBCRYPT    = implicit
   USE_LINUX_SPLICE= implicit
   USE_LINUX_TPROXY= implicit