Fixes "multiple definition" errors when building with GCC 10+.

Based on <https://bugs.gentoo.org/show_bug.cgi?id=706706>.
--- a/fig2dev/dev/gensvg.c	2010-07-01 22:41:16.000000000 +0200
+++ b/fig2dev/dev/gensvg.c	2022-02-27 20:02:33.379945500 +0100
@@ -230,10 +230,12 @@
 };
 
 /* arrowhead arrays */
-Point   points[50], fillpoints[50], clippoints[50];
-int     npoints, nfillpoints, nclippoints;
-int     arrowx1, arrowy1;	/* first point of object */
-int     arrowx2, arrowy2;	/* second point of object */
+Point   fillpoints[50];
+int     nfillpoints;
+extern Point   points[50], clippoints[50];
+extern int     npoints, nclippoints;
+extern int     arrowx1, arrowy1;	/* first point of object */
+extern int     arrowx2, arrowy2;	/* second point of object */
 
 static int tileno=0; /* number of current tile */ 
 
--- a/fig2dev/fig2dev.h	2010-03-16 19:53:20.000000000 +0100
+++ b/fig2dev/fig2dev.h	2022-02-27 19:56:06.072253991 +0100
@@ -126,7 +126,7 @@
 extern char	*prog, *from, *to;
 extern char	*name;
 extern double	font_size;
-Boolean	correct_font_size;	/* use correct font size */
+extern Boolean	correct_font_size;	/* use correct font size */
 extern double	mag, fontmag;
 extern FILE	*tfp;