gnu: plotutils: Fix build on non-x86_64 architectures.

* gnu/packages/patches/plotutils-spline-test.patch: Add patch.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/plotutils.scm (plotutils)[patches]: Apply it.

gnu/packages/patches/plotutils-spline-test.patch

@@ -0,0 +1,34 @@
+Description: Work around i386 testsuite error
+Author: Andreas Metzler <ametzler@debian.org>
+Origin: vendor
+Bug: http://lists.gnu.org/archive/html/bug-plotutils/2017-02/msg00001.html
+Bug-Debian: https://bugs.debian.org/856134
+Forwarded: http://lists.gnu.org/archive/html/bug-plotutils/2017-02/msg00002.html
+Last-Update: 2017-02-25
+
+--- a/test/spline.test
++++ b/test/spline.test
+@@ -3,12 +3,21 @@
+ echo 0 1 0.5 0.75 -0.25 0 | ../spline/spline -a -T 1 -n 120 \
+ 	--precision 5 -p >spline.out
+ 
++# Work around testsuite error
++# http://lists.gnu.org/archive/html/bug-plotutils/2016-04/msg00000.html
++#
+ # work around line end problems in installations with DJGPP under DOS
+-tr -d '\015' < spline.out > spline.dos
++tr -d '\015' < spline.out \
++	| head -n 120 | tail -n 119 \
++	> spline.dos
+ 
+-if cmp -s $SRCDIR/spline.xout spline.dos
++# cannot patch spline.xout, it is used as input for graph.test.
++head -n 120 $SRCDIR/spline.xout | tail -n 119 > spline.xout.4comp
++
++if cmp -s spline.xout.4comp spline.dos
+ 	then retval=0;
+ 	else retval=1;
+ 	fi;
+ 
++rm -f spline.xout.4comp
+ exit $retval