Fix --enable-systemd builds on systems with libsystemd but not systemd

Fixes bug 16164; bugfix on 0.2.6.3-alpha. Patch from Peter Palfrader.

changes/bug16164

+  o Minor bugfixes (compilation):
+    - Build with --enable-systemd correctly when libsystemd is installed,
+      but systemd is not. Fixes bug 16164, bugfix on 0.2.6.3-alpha. Patch
+      from Peter Palfrader.

configure.ac

@@ -131,7 +131,7 @@ if test x$have_systemd = xyes; then
     AC_DEFINE(HAVE_SYSTEMD,1,[Have systemd])
     TOR_SYSTEMD_CFLAGS="${SYSTEMD_CFLAGS}"
     TOR_SYSTEMD_LIBS="${SYSTEMD_LIBS}"
-    PKG_CHECK_MODULES(SYSTEMD209, [systemd >= 209],
+    PKG_CHECK_MODULES(LIBSYSTEMD209, [libsystemd >= 209],
          [AC_DEFINE(HAVE_SYSTEMD_209,1,[Have systemd v209 or more])], [])
 fi
 AC_SUBST(TOR_SYSTEMD_CFLAGS)