configure.ac: Only enable enable_simple_egl_clients if enable_egl is yes

This simplifies a few checks and makes sure we only build the
subsurfaces client if EGL is enabled.

https://bugs.freedesktop.org/show_bug.cgi?id=68727

configure.ac

@@ -276,9 +276,9 @@ fi
 AC_ARG_ENABLE(simple-egl-clients,
               AS_HELP_STRING([--disable-simple-egl-clients],
                              [do not build the simple EGL clients]),,
-              enable_simple_egl_clients=yes)
-AM_CONDITIONAL(BUILD_SIMPLE_EGL_CLIENTS, test "x$enable_simple_egl_clients" = "xyes" -a "x$enable_egl" = "xyes")
-if test x$enable_simple_egl_clients = xyes -a x$enable_egl = xyes; then
+              enable_simple_egl_clients="$enable_egl")
+AM_CONDITIONAL(BUILD_SIMPLE_EGL_CLIENTS, test "x$enable_simple_egl_clients" = "xyes")
+if test x$enable_simple_egl_clients = xyes; then
   PKG_CHECK_MODULES(SIMPLE_EGL_CLIENT,
                     [egl >= 7.10 glesv2 wayland-client wayland-egl wayland-cursor])
 fi