build: don't install systemd unit file outside of prefix

Problem: make distcheck fails in an attempt to install
the systemd unit file into the actual system directory,
which is not writeable.

Bring over a fix from flux-core (courtesy @grondo):
flux-framework/flux-core@35ee727

His comments:
  When --with-systemdsystemunitdir is not set explicitly, prepend
  the pkg-config systemdsystemunitdir with ${prefix}, so that the
  unit file is not installed outside of --prefix.

  Distro packages should therefore use --with-systemdsystemunitdir
  explicitly, in the case where the unit file directory *is* outside
  of prefix (e.g. /lib/systemd/system when --prefix=/usr).

Based on the spec files in the examples subdir, I think we are
likely already setting this explicitly when we build RPMs for TOSS.

config/systemd.m4

@@ -33,7 +33,7 @@ AC_DEFUN([RRA_WITH_SYSTEMD_UNITDIR],
     [AS_HELP_STRING([--with-systemdsystemunitdir=DIR],
         [Directory for systemd service files])],
     [],
-    [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
+    [with_systemdsystemunitdir=\${prefix}$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
  AS_IF([test x"$with_systemdsystemunitdir" != xno],
     [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
  AM_CONDITIONAL([HAVE_SYSTEMD],