From a40a25e14df23ecb04b03d66b686f0d670400243 Mon Sep 17 00:00:00 2001
From: "Peter W. Draper" <p.w.draper@durham.ac.uk>
Date: Wed, 25 Apr 2018 18:35:53 +0100
Subject: [PATCH] Need to clear settings when lib is not found and not
 specified

---
 configure.ac | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/configure.ac b/configure.ac
index 9eca25356..5ad10e872 100644
--- a/configure.ac
+++ b/configure.ac
@@ -444,6 +444,10 @@ if test "x$with_gsl" != "xno"; then
          AC_MSG_ERROR(something is wrong with the GSL library!), $GSL_LIBS)
       have_gsl="yes"
    fi
+   if test "$have_gsl" = "no"; then
+      GSL_LIBS=""
+      GSL_INCS=""
+   fi
 fi
 AC_SUBST([GSL_LIBS])
 AC_SUBST([GSL_INCS])
@@ -574,6 +578,10 @@ if test "x$with_fftw" != "xno"; then
          AC_MSG_ERROR(something is wrong with the FFTW library!), $FFTW_LIBS)
       have_fftw="yes"
    fi      
+   if test "$have_fftw" = "no"; then
+      FFTW_LIBS=""
+      FFTW_INCS=""
+   fi
 fi
 AC_SUBST([FFTW_LIBS])
 AC_SUBST([FFTW_INCS])
-- 
2.26.2