m4: grab newer upstream m4 (for code-coverage)

Grab some newer version of the autotools m4 archive, as we need those to
setup code-coverage.

Remove some unneeded ones.

m4/ax_ac_append_to_file.m4

@@ -0,0 +1,32 @@
+# ===========================================================================
+#   https://www.gnu.org/software/autoconf-archive/ax_ac_append_to_file.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_AC_APPEND_TO_FILE([FILE],[DATA])
+#
+# DESCRIPTION
+#
+#   Appends the specified data to the specified Autoconf is run. If you want
+#   to append to a file when configure is run use AX_APPEND_TO_FILE instead.
+#
+# LICENSE
+#
+#   Copyright (c) 2009 Allan Caffee <allan.caffee@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 10
+
+AC_DEFUN([AX_AC_APPEND_TO_FILE],[
+AC_REQUIRE([AX_FILE_ESCAPES])
+m4_esyscmd(
+AX_FILE_ESCAPES
+[
+printf "%s" "$2" >> "$1"
+])
+])