The second attempt at updating the AX_* macros in commit
7ae86729 seem to have again broken the build:
  autoreconf: running: aclocal --force -I m4 -I libgd ${ACLOCAL_FLAGS}
  configure.ac:21: error: macro AX_APPEND_LINK_FLAGS is not defined;
    is a m4 file missing?
  m4/ax_require_defined.m4:35: AX_REQUIRE_DEFINED is expanded from...
  m4/ax_compiler_flags_ldflags.m4:30: AX_COMPILER_FLAGS_LDFLAGS is
    expanded from...
  m4/ax_compiler_flags.m4:113: AX_COMPILER_FLAGS is expanded from...
  configure.ac:21: the top level

It is not clear why the build didn't break while testing commit
7ae86729 or any of the subsequent ones after that. Continuous
and the nightly Flatpak builds have also been chugging along without
any problems. It's a mystery.

Anyway, including the definition of AX_APPEND_LINK_FLAGS seems like
the obvious thing to do.