configure: make clear-memory default but only WARN if backend unsuppo…

…rted

... instead of previous ERROR.

configure.ac

@@ -202,12 +202,12 @@ 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" = "yes"; then
+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])
+    AC_MSG_WARN([secure clearing/zeroing of memory is not supported by the selected crypto backend])
     enable_clear_memory=unsupported
   fi
 else