exo-csource is necessary only in maintainer mode

--- lightdm-gtk-greeter-2.0.2/configure.ac.1	2017-08-29 18:26:57.845778728 +0000
+++ lightdm-gtk-greeter-2.0.2/configure.ac	2017-08-29 18:28:36.183599352 +0000
@@ -48,10 +48,12 @@
 )
 PKG_CHECK_MODULES([LIBX11], [x11])
 
+if test $USE_MAINTAINER_MODE = yes; then
 AC_CHECK_PROG([HAVE_EXO_CSOURCE], [exo-csource], [yes])
 if test x"$HAVE_EXO_CSOURCE" != xyes; then
   AC_MSG_FAILURE([exo-csource not installed])
 fi
+fi
 
 dnl ###########################################################################
 dnl Optional dependencies