Fix an optimization issue seen with Studion 12.1 with Solaris 11.3 when
compiling with -xO4. The problem has been fixed in Studio 12.4, so the
issue will go away when the Solaris 11.X release updates to that in its CBE.

This patch will not be sent upstream.

--- gsl-1.16/interpolation/linear.c.orig	2015-06-01 15:58:46.278682889 -0700
+++ gsl-1.16/interpolation/linear.c	2015-06-01 16:28:26.000366577 -0700
@@ -157,7 +157,7 @@
   *result = 0.0;
   
   /* interior intervals */
-  for(i=index_a; i<=index_b; i++) {
+  for(i=index_a; i<index_b+1; i++) {
     const double x_hi = x_array[i + 1];
     const double x_lo = x_array[i];
     const double y_lo = y_array[i];