Fix use of OPENDHT_SYSTEMD_UNIT_FILE_LOCATION

For some reason 1778f640 reverted part of 4d70c214

This commit fixes that.

tools/CMakeLists.txt

@@ -39,8 +39,14 @@ endif ()
 install (TARGETS dhtnode dhtscanner dhtchat RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 
 if (OPENDHT_SYSTEMD)
+    if (NOT DEFINED OPENDHT_SYSTEMD_UNIT_FILE_LOCATION)
         execute_process(COMMAND ${PKG_CONFIG_EXECUTABLE} systemd --variable=systemdsystemunitdir
                         OUTPUT_VARIABLE SYSTEMD_UNIT_INSTALL_DIR)
+        message("-- Using Systemd unit installation directory by pkg-config: " ${SYSTEMD_UNIT_INSTALL_DIR})
+    else()
+        message("-- Using Systemd unit installation directory requested: " ${OPENDHT_SYSTEMD_UNIT_FILE_LOCATION})
+        set(SYSTEMD_UNIT_INSTALL_DIR ${OPENDHT_SYSTEMD_UNIT_FILE_LOCATION})
+    endif()
     string(REGEX REPLACE "[ \t\n]+" "" SYSTEMD_UNIT_INSTALL_DIR "${SYSTEMD_UNIT_INSTALL_DIR}")
     set (systemdunitdir "${SYSTEMD_UNIT_INSTALL_DIR}")