Fix s390x build failure

gnuplot-signed-char.patch

@@ -0,0 +1,39 @@
+From 10995e7043211c793d545e7b2cc95acb4760e479 Mon Sep 17 00:00:00 2001
+From: Ethan A Merritt <merritt@u.washington.edu>
+Date: Sun, 17 Oct 2021 14:28:41 -0700
+Subject: [PATCH] arm and s390 need "signed char"
+
+The default is unsigned, which breaks array bound checks in the
+vplot.c isosurface code.
+Bugs #2467 #2450
+---
+ src/marching_cubes.h | 2 +-
+ src/qt_table.h       | 2 +-
+ 2 files changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/src/marching_cubes.h b/src/marching_cubes.h
+index 4f7264f93..4f1a5a872 100644
+--- a/src/marching_cubes.h
++++ b/src/marching_cubes.h
+@@ -58,7 +58,7 @@ static const short cube_edge_flags[256]=
+          First official public domain release
+               Version 3.00 dated 8-6-94
+  */
+-static char triangle_table[256][13]=
++static signed char triangle_table[256][13]=
+ {
+     {-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1},
+     { 8, 3, 0,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1},
+diff --git a/src/qt_table.h b/src/qt_table.h
+index 9e4ebf8e8..747ef8ec1 100644
+--- a/src/qt_table.h
++++ b/src/qt_table.h
+@@ -10,7 +10,7 @@
+  *     collapsed them into quadrangles.
+  *     So there is room for improvement in this table.
+  */ 
+-static char qt_table[256][13] = 
++static signed char qt_table[256][13] =
+ {
+     {  -1, -1, -1, -1,  -1, -1, -1, -1,  -1, -1, -1, -1,  -1},
+     {   8,  3,  0, -1,  -1, -1, -1, -1,  -1, -1, -1, -1,  -1},

gnuplot.spec

@@ -39,6 +39,8 @@ Patch6: gnuplot-5.4.2-no-lena.patch
 Patch7: gnuplot-5.2.2-doc.patch
 Patch8: gnuplot-5.2.8-cmd-opts.patch
 Patch9: gnuplot-config.patch
+# Upstream patch to fix s390x build failure
+Patch20: gnuplot-signed-char.patch
 
 Requires: %{name}-common = %{version}-%{release}
 Requires: dejavu-sans-fonts
@@ -175,6 +177,7 @@ plotting tool.
 %patch7 -p1 -b .doc
 %patch8 -p1 -b .cmd-opts
 %patch9 -p1 -b .config
+%patch20 -p1 -b .signed-char
 sed -i -e 's:"/usr/lib/X11/app-defaults":"%{x11_app_defaults_dir}":' src/gplt_x11.c
 chmod 644 src/getcolor.h
 chmod 644 demo/html/webify.pl