[m4-common] Add AX_GENERATE_CHANGELOG macro

[Philip Withnall <pwithnall src gnome org>]

commit 04f8f1a0de7401191e310eec825a31522a501e59
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Tue Jul 12 13:39:17 2016 +0100

    Add AX_GENERATE_CHANGELOG macro
    
    It’s now needed by libgdata, and is a generally useful macro to have.
    
    https://www.gnu.org/software/autoconf-archive/ax_generate_changelog.html

 Makefile.am |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index 66f3845..7ae8087 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,6 +19,7 @@ dist_aclocal_DATA = \
        autoconf-archive/m4/ax_compiler_flags_gir.m4                    \
        autoconf-archive/m4/ax_cxx_compile_stdcxx.m4                    \
        autoconf-archive/m4/ax_cxx_compile_stdcxx_11.m4                 \
+       autoconf-archive/m4/ax_generate_changelog.m4                    \
        autoconf-archive/m4/ax_pkg_check_modules.m4                     \
        autoconf-archive/m4/ax_prog_gjs.m4                              \
        autoconf-archive/m4/ax_require_defined.m4                       \