[balsa/gtk3] Fix building when _FORTIFY_SOURCE is predefined

[Peter Bloomfield <peterb src gnome org>]

commit e2433a6c346e74c04ec95537bc10bb210bd5716e
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Sat Jul 23 20:35:14 2016 -0400

    Fix building when _FORTIFY_SOURCE is predefined
    
        * configure.ac: undefine _FORTIFY_SOURCE before defining it to
        have the value 2.

 ChangeLog    |    7 +++++++
 configure.ac |    2 +-
 2 files changed, 8 insertions(+), 1 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 71f294b..f7465fb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2016-07-23  Peter Bloomfield  <pbloomfield bellsouth net>
+
+       Fix building with toolchains that predefine _FORTIFY_SOURCE
+
+       * configure.ac: undefine _FORTIFY_SOURCE before defining it to
+       have the value 2.
+
 2016-06-30  Peter Bloomfield  <pbloomfield bellsouth net>
 
        Use Gspell-1.2 API
diff --git a/configure.ac b/configure.ac
index 63ad46c..e5b8030 100644
--- a/configure.ac
+++ b/configure.ac
@@ -786,7 +786,7 @@ if test -d "${srcdir}/.git" ; then
 -DGTK_DISABLE_SINGLE_INCLUDES \
 -DNM_DISABLE_DEPRECATED \
 -DPANGO_DISABLE_DEPRECATED \
--D_FORTIFY_SOURCE=2 \
+-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 \
 "
 
     if test x"$use_html_widget" != xgtkhtml2 ; then