[gnome-flashback] Fix build with gettext 0.20

[Alberts Muktupāvels <muktupavels src gnome org>]

commit bf1c61f929749407a1faeade7da39e8918834d76
Author: Balló György <ballogyor gmail com>
Date:   Sun Mar 8 19:00:45 2020 +0000

    Fix build with gettext 0.20
    
    Without this definition, the build fails with the following error:
    *** error: gettext infrastructure mismatch: using a Makefile.in.in from gettext version 0.19 but the 
autoconf macros are from gettext version 0.20

 configure.ac | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 3de31cf..e9950bb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -70,7 +70,8 @@ dnl Internationalization
 dnl **************************************************************************
 
 AM_GNU_GETTEXT([external])
-AM_GNU_GETTEXT_VERSION([0.19.4])
+AM_GNU_GETTEXT_VERSION([0.19.6])
+AM_GNU_GETTEXT_REQUIRE_VERSION([0.19.6])
 
 GETTEXT_PACKAGE=gnome-flashback
 AC_SUBST([GETTEXT_PACKAGE])