do not override user supplied LIBJPEG_LIBS so we can use the one in WORKDIR

--- poppler/configure.orig	2016-11-02 13:58:32.530617038 +0100
+++ poppler/configure	2016-11-02 13:58:06.233617689 +0100
@@ -21377,8 +21377,9 @@
           ac_save_USER_INCLUDES="$USER_INCLUDES"
   ac_save_USER_LDFLAGS="$USER_LDFLAGS"
   USER_INCLUDES="$USER_INCLUDES $LIBJPEG_CFLAGS"
-  USER_LDFLAGS="$USER_LDFLAGS $LIBJPEG_CFLAGS"
+  USER_LDFLAGS="$USER_LDFLAGS $LIBJPEG_LIBS"
 
+if test -z "$LIBJPEG_LIBS"; then
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for libjpeg6b" >&5
 $as_echo_n "checking for libjpeg6b... " >&6; }
@@ -21489,6 +21490,7 @@
 
 
 fi
+fi
 
 
 ac_save_CPPFLAGS="$CPPFLAGS"