Correct method to determine systemd unit path

As recomended by debain maintainers and
http:https://www.freedesktop.org/software/systemd/man/daemon.html#Installing%20Systemd%20Service%20Files

Makefile.am

@@ -61,6 +61,6 @@ if INSTALL_SYSTEMV
  [ -f $(DESTDIR)/etc/init.d/shairport-sync ] || cp scripts/shairport-sync $(DESTDIR)/etc/init.d/
 endif
 if INSTALL_SYSTEMD
- [ -e $(DESTDIR)/usr/lib/systemd/system ] || mkdir -p $(DESTDIR)/usr/lib/systemd/system
- cp scripts/shairport-sync.service $(DESTDIR)/usr/lib/systemd/system
+ [ -e $(DESTDIR)$(systemdsystemunitdir) ] || mkdir -p $(DESTDIR)$(systemdsystemunitdir)
+ cp scripts/shairport-sync.service $(DESTDIR)$(systemdsystemunitdir)
 endif

configure.ac

@@ -213,6 +213,22 @@ AC_ARG_WITH(dns_sd, [ --with-dns_sd = choose dns_sd mDNS support], [
  AC_SEARCH_LIBS([DNSServiceRefDeallocate], [dns_sd], , AC_MSG_ERROR(dns_sd support requires the dns_sd library!))], )
 AM_CONDITIONAL([USE_DNS_SD], [test "x$HAS_DNS_SD" = "x1"])
 
+# Find systemd unit dir
+AC_ARG_WITH([systemdsystemunitdir],
+ [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
+ [with_systemdsystemunitdir=auto])
+AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
+ def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)
+
+ AS_IF([test "x$def_systemdsystemunitdir" = "x"],
+ [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
+ [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
+ with_systemdsystemunitdir=no],
+ [with_systemdsystemunitdir="$def_systemdsystemunitdir"])])
+AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
+ [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
+AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])
+
 # Checks for header files.
 AC_HEADER_STDC
 AC_CHECK_HEADERS([getopt_long.h])