Fix build with automake 1.16.5. Thanks for Fabrice Fontaine for this.

configure.ac

@@ -2,8 +2,8 @@
 # Process this file with autoconf to produce a configure script.
 
 AC_PREREQ([2.50])
-AC_INIT([shairport-sync], [3.3.8], [4265913+mikebrady@users.noreply.github.com])
-AM_INIT_AUTOMAKE
+AC_INIT([shairport-sync], [3.3.9], [4265913+mikebrady@users.noreply.github.com])
+AM_INIT_AUTOMAKE([subdir-objects])
 AC_CONFIG_SRCDIR([shairport.c])
 AC_CONFIG_HEADERS([config.h])
 AC_PROG_RANLIB
@@ -302,7 +302,6 @@ AM_CONDITIONAL([USE_PA], [test "x$with_pa" = "xyes"])
 # Look for Convolution flag
 AC_ARG_WITH(convolution, [AS_HELP_STRING([--with-convolution],[choose audio DSP convolution support])])
 if test "x$with_convolution" = "xyes" ; then
-  AM_INIT_AUTOMAKE([subdir-objects])
   AC_DEFINE([CONFIG_CONVOLUTION], 1, [Include audio DSP convolution support.])
   AC_CHECK_LIB([sndfile], [sf_open], , AC_MSG_ERROR(Convolution support requires the sndfile library -- libsndfile1-dev suggested!))
 fi