wincng: Added explicit clear memory feature to WinCNG backend

This re-introduces the original feature proposed during
the development of the WinCNG crypto backend. It still needs
to be added to libssh2 itself and probably other backends.

Memory is cleared using the function SecureZeroMemory which is
available on Windows systems, just like the WinCNG backend.

configure.ac

@@ -97,6 +97,7 @@ AC_ARG_WITH(libz,
   use_libz=$withval,use_libz=auto)
 
 found_crypto=none
+support_clear_memory=no
 
 # Look for OpenSSL
 if test "$found_crypto" = "none" && test "$use_openssl" != "no"; then
@@ -150,6 +151,7 @@ if test "$ac_cv_libbcrypt" = "yes"; then
     LIBS="$LIBS -lcrypt32"
   fi
   found_crypto="Windows Cryptography API: Next Generation"
+  support_clear_memory=yes
 fi
 AM_CONDITIONAL(WINCNG, test "$ac_cv_libbcrypt" = "yes")
 
@@ -197,6 +199,26 @@ if test "$GEX_NEW" != "no"; then
   AC_DEFINE(LIBSSH2_DH_GEX_NEW, 1, [Enable newer diffie-hellman-group-exchange-sha1 syntax])
 fi
 
+AC_ARG_ENABLE(clear-memory,
+  AC_HELP_STRING([--disable-clear-memory],[Disable clearing of memory before being freed]),
+  [CLEAR_MEMORY=$enableval])
+if test "$CLEAR_MEMORY" != "no"; then
+  if test "$support_clear_memory" = "yes"; then
+    AC_DEFINE(LIBSSH2_CLEAR_MEMORY, 1, [Enable clearing of memory before being freed])
+    enable_clear_memory=yes
+  else
+    AC_MSG_ERROR([secure clearing/zeroing of memory is not supported by the selected crypto backend])
+    enable_clear_memory=unsupported
+  fi
+else
+  if test "$support_clear_memory" = "yes"; then
+    enable_clear_memory=no
+  else
+    AC_MSG_WARN([secure clearing/zeroing of memory is not supported by the selected crypto backend])
+    enable_clear_memory=unsupported
+  fi
+fi
+
 dnl ************************************************************
 dnl option to switch on compiler debug options
 dnl
@@ -362,6 +384,7 @@ AC_MSG_NOTICE([summary of build options:
   Compiler flags:   ${CFLAGS}
   Library types:    Shared=${enable_shared}, Static=${enable_static}
   Crypto library:   ${found_crypto}
+  Clear memory:     $enable_clear_memory
   Debug build:      $enable_debug
   Build examples:   $build_examples
   Path to sshd:     $ac_cv_path_SSHD (only for self-tests)