Squashed commit of the following:

Add three parameters to the backend play() function call -- (1) a flag indicating whether the samples are timed or not. If timed, (2) the timestamp and (3) the local time at which the first frame should be heard.
    Update missing pipewire library message.
    Makefile.am fix.
    Update the SHM version and fix the SHM name so that it works with FreeBSD.
    Remove redundant (?) AC_HEADER_STDC check.
    Fix compilation and installation under FreeBSD -- changes to allow building in a separate directory broke the FreeBSD build process.
    Add AC_CHECK_LIB for gcrypt in case PKG_CHECK_MODULES fails to find it.
    Find gcrypt using pkg-config
    Add code to detect when the RTSP channel goes idle for a period, and use it to reset clocks and timings, etc. so that SPS resumes correctly where the source has gone to sleep while playing a realtime stream and has subsequently woken up.
    Don't exit if a UDP Clock Control packet is empty. Also check minimum packet size.
    Remove some very experimental code which may cause memory and port leaks.
    Strip file path from the filename used in debug messages, information messages, warnings and fatal error messages.
    BB fix to Makefile.am -- overwriting the DBUS stuff with MPRIS definitions, duh.
    Fix some problems building on FreeBSD and tidy up the use of the "sed" editor.
    Fix some problems building on picore.
    Fix an uninitialised variable
    start using the "nqptp" SMI

dacp.c

@@ -206,7 +206,8 @@ int dacp_send_command(const char *command, char **body, ssize_t *bodysize) {
       pthread_cleanup_push(addrinfo_cleanup, (void *)&res);
       // only do this one at a time -- not sure it is necessary, but better safe than sorry
 
-      // int mutex_reply = sps_pthread_mutex_timedlock(&dacp_conversation_lock, 2000000, command, 1);
+      // int mutex_reply = sps_pthread_mutex_timedlock(&dacp_conversation_lock, 2000000, command,
+      // 1);
       int mutex_reply = debug_mutex_lock(&dacp_conversation_lock, 2000000, 1);
       // int mutex_reply = pthread_mutex_lock(&dacp_conversation_lock);
       if (mutex_reply == 0) {