MXS-2057 Do not require systemd libraries

Exclude systemd usage if the library is not installed.
Only excluding what is necessary. This keeps the object size the
same and still compiles most of the code.

maxutils/maxbase/src/CMakeLists.txt

@@ -15,9 +15,9 @@ add_library(maxbase STATIC
   average.cc
   )
 
-target_link_libraries(maxbase
-  systemd
-)
+if(HAVE_SYSTEMD)
+target_link_libraries(maxbase systemd)
+endif()
 
 set_target_properties(maxbase PROPERTIES VERSION "1.0.0" LINK_FLAGS -Wl,-z,defs)
 add_subdirectory(test)