configure: use --disable-tests, since default is enabled

Add a nice help text, too.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

configure.ac

@@ -149,7 +149,10 @@ AC_ARG_ENABLE(tablet-shell, [  --enable-tablet-shell],,
 AM_CONDITIONAL(ENABLE_TABLET_SHELL,
 	       test x$enable_tablet_shell == xyes)
 
-AC_ARG_ENABLE(tests, [  --enable-tests],,enable_tests=yes)
+AC_ARG_ENABLE(tests,
+	      AS_HELP_STRING([--disable-tests],
+			     [do not build unit tests [default=enabled]]),,
+	      enable_tests=yes)
 AM_CONDITIONAL(BUILD_TESTS, test x$enable_tests == xyes)
 
 if test "x$GCC" = "xyes"; then