Add configure option for ReGIS support.
https://bugs.gentoo.org/643888

--- gnuplot-5.2.2-orig/configure.ac
+++ gnuplot-5.2.2/configure.ac
@@ -1057,6 +1057,13 @@
   AC_DEFINE(HAVE_MIF,1,
           [ Define to include support for mif terminal ],))
 
+dnl ReGIS terminal
+AC_ARG_WITH(regis,dnl
+[  --with-regis            ReGIS terminal])
+AS_IF([test "x${with_regis}" = "xyes"],
+  AC_DEFINE(HAVE_REGIS,1,
+          [ Define to include support for ReGIS terminal ],))
+
 AC_ARG_WITH(cairo,dnl
 [  --without-cairo         cairo-based terminals (default enabled)],,
   [test -z "${with_cairo}" && with_cairo=yes])
@@ -1350,6 +1357,12 @@
   AC_MSG_RESULT([  mif terminal: no   (use --with-mif to enable)])
 fi
 
+if test "$with_regis" = yes; then
+  AC_MSG_RESULT([  ReGIS terminal: yes])
+else
+  AC_MSG_RESULT([  ReGIS terminal: no (use --with-regis to enable)])
+fi
+
 if test "$is_msdos" = yes; then
   AC_MSG_RESULT([  svga terminal (MSDOS/djgpp): yes])
 fi
--- gnuplot-5.2.2-orig/src/term.h
+++ gnuplot-5.2.2/src/term.h
@@ -182,7 +182,7 @@
 #endif
 
 /* REGIS graphics language */
-#ifdef VMS
+#if defined(VMS) || defined(HAVE_REGIS)
 # include "regis.trm"
 #endif