Fix linking to systemd when elogind is used instead

This is mainly useful for non-glibc systems. Yes, systemd doesn't even run
on non-glibc systems, but elogind does and this way it links to both systemd
and elogind.

However due to switching the way we link to systemd, we now also make
sure systemd is actually installed on the system before we even try to
compile, thus preventing compiler errors when systemd isn't present.

src/launcherlib/CMakeLists.txt

@@ -2,7 +2,12 @@ include(GNUInstallDirs)
 
 set(COMMON ${CMAKE_HOME_DIRECTORY}/src/common)
 
-include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${COMMON})
+# Find systemd
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(SYSTEMD "libsystemd" REQUIRED)
+
+# Set include dirs
+include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${SYSTEMD_INCLUDE_DIRS} ${COMMON})
 
 # Hide all symbols except the ones explicitly exported in the code (like main())
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden")
@@ -16,12 +21,15 @@ set(HEADERS appdata.h booster.h connection.h daemon.h logger.h launcherlib.h
 
 # Set libraries to be linked. Shared libraries to be preloaded are not linked in anymore,
 # but dlopen():ed and listed in src/launcher/preload.h instead.
-link_libraries(${LIBDL} "-L/lib -lsystemd -lcap")
+link_libraries(${LIBDL} "-L/lib -lcap")
 
 # Set executable
 add_library(applauncherd MODULE ${SRC} ${MOC_SRC})
 set_target_properties(applauncherd PROPERTIES VERSION 0.1 SOVERSION 0)
 
+target_link_libraries(applauncherd ${SYSTEMD_LIBRARIES})
+
+
 # Add install rule
 install(TARGETS applauncherd DESTINATION ${CMAKE_INSTALL_FULL_LIBDIR})
 install(FILES ${HEADERS} DESTINATION /usr/include/applauncherd