Set with_cmocka=no by default

configure

@@ -22538,7 +22538,9 @@ fi
 # Check whether --with-cmocka was given.
 if test "${with_cmocka+set}" = set; then :
-  withval=$with_cmocka;
+  withval=$with_cmocka; :
+else
+  with_cmocka=no
 fi

configure.in

@@ -4551,7 +4551,8 @@ AC_SUBST([LIBIDN2_LIBS])
 #
 
 AC_ARG_WITH([cmocka],
-	    [AS_HELP_STRING([--with-cmocka=no],[enable cmocka based tests (default is no)])])
+	    [AS_HELP_STRING([--with-cmocka=no],[enable cmocka based tests (default is no)])],
+	    [:],[with_cmocka=no])
 
 AS_CASE([$with_cmocka],
 	[no],[:],