disallow invalid values for WITH_SYSTEMD cmake variable.

cmake/systemd.cmake

@@ -76,9 +76,11 @@ MACRO(CHECK_SYSTEMD)
         UNSET(HAVE_SYSTEMD_SD_NOTIFYF)
         MESSAGE(STATUS "Systemd features not enabled")
         IF(WITH_SYSTEMD STREQUAL "yes")
-          MESSAGE(FATAL_ERROR "Requested WITH_SYSTEMD=YES however no dependencies installed/found")
+          MESSAGE(FATAL_ERROR "Requested WITH_SYSTEMD=yes however no dependencies installed/found")
         ENDIF()
       ENDIF()
+    ELSEIF(NOT WITH_SYSTEMD STREQUAL "no")
+      MESSAGE(FATAL_ERROR "Invalid value for WITH_SYSTEMD. Must be 'yes', 'no', or 'auto'.")
     ENDIF()
   ENDIF()
 ENDMACRO()