autotools: Add an option to install demo clients

This patch adds a configure option which will enable
user to install demo clients if desired. It is disabled
by default.

v2: Remove AC_DEFINE as it is not necesary
diff --git a/configure.ac b/configure.ac
index 5b110e0..f9f1c53 100644
--- a/configure.ac
+++ b/configure.ac
@@ -383,6 +383,12 @@
 AC_DEFINE_UNQUOTED([WESTON_NATIVE_BACKEND], ["$WESTON_NATIVE_BACKEND"],
 		   [The default backend to load, if not wayland nor x11.])
 
+AC_ARG_ENABLE(demo-clients,
+              AS_HELP_STRING([--enable-demo-clients],
+                             [install demo clients built with weston]),,
+              enable_demo_clients=no)
+AM_CONDITIONAL(ENABLE_DEMO_CLIENTS, [test "x$enable_demo_clients" = "xyes"])
+
 PKG_CHECK_MODULES(LCMS, lcms2,
                   [have_lcms=yes], [have_lcms=no])
 if test "x$have_lcms" = xyes; then