Add support for generating coverage reports

This change adds autoconf/automake support for building all of mosh
with gcov, and generates an lcov html report. This allows seeing which
parts ofthe source tree have good test coverage, and which can be
shored up. Eventually, it would be good to hook this up to Github
Actions to be generated automatically.

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"
+])
+])