Patch from Debian: "use system gsl", see
https://sources.debian.org/patches/k2pdfopt/2.55%2Bds-2/0008-willuslib-gslpolyfit.c-use-system-gsl.patch/

Index: willuslib/gslpolyfit.c
--- willuslib/gslpolyfit.c.orig
+++ willuslib/gslpolyfit.c
@@ -24,7 +24,7 @@
 #include <stdlib.h>
 #include "willus.h"
 #ifdef HAVE_GSL_LIB
-#include <gsl.h>
+#include <gsl/gsl_multifit.h>
 #endif
 
 void gslpolyfit(double *x,double *y0,int n,int d,double *c0)
