title: Cmake variables for systemd are empty
body: <p>I want to start/stop a systemd service from within my C++ project. Therefore I like to use the systemd lib and headers provided. I'm using cmake for makefile generation. I'm trying to get systemd in cmake via</p>
  
  <pre><code>pkg_check_modules(SDBUS REQUIRED systemd)
  </code></pre>
  
  <p>which actually works as</p>
  
  <p><code>message(STATUS "FOUND" ${SDBUS_FOUND})</code>  returns 1.</p>
  
  <p>However, all the other necessary vars created by cmake like</p>
  
  <pre><code>message(STATUS "LIBS" ${SDBUS_LIBRARIES})
  message(STATUS "LIB_DIRS" ${SDBUS_LIBRARY_DIRS})
  message(STATUS "LDFLAGS" ${SDBUS_LDFLAGS})
  message(STATUS "LDFLAGS_OTHER" ${SDBUS_LDFLAGS_OTHER})
  message(STATUS "INCLUDE DIRS" ${SDBUS_INCLUDE_DIRS})
  message(STATUS "CFLAGS" ${SDBUS_CFLAGS})
  message(STATUS "CFLAGS_OTHER" ${SDBUS_CFLAGS_OTHER})
  </code></pre>
  
  <p>are empty. Which leads to a linker error when compiling the project. Indeed when calling</p>
  
  <pre><code>pkg-config --libs systemd
  </code></pre>
  
  <p>it's empty as well. Anyone has any idea on how to solve that and why all these information is not available for systemd?</p>