MDEV-19481 mariadb-10.3.15-linux-systemd-x86_64/bin/mysqld: /lib64/li…

…bstdc++.so.6: version `GLIBCXX_3.4.20' and 'GLIBCXX_3.4.21' not found

don't ignore LIBSYSTEMD_LDFLAGS when trying out systemd

(and put them in LIBSYSTEMD, because MYSQLD_LINK_FLAGS has stuff like
-lmtmalloc and is used only for mysqld, not for, say, explain_filename-t)

LIBSYSTEMD_LDFLAGS come from libsystemd.pc via pkg-config

cmake/systemd.cmake

@@ -27,13 +27,9 @@ MACRO(CHECK_SYSTEMD)
           pkg_search_module(LIBSYSTEMD libsystemd libsystemd-daemon)
         ENDIF()
         IF(HAVE_DLOPEN)
-          SET(LIBSYSTEMD ${LIBSYSTEMD_LIBRARIES})
-          #SET(CMAKE_REQUIRED_FLAGS ${LIBSYSTEMD_CFLAGS})
-          SET(MYSQLD_LINK_FLAGS "${MYSQLD_LINK_FLAGS} ${LIBSYSTEMD_LDFLAGS}")
+          SET(LIBSYSTEMD ${LIBSYSTEMD_LDFLAGS} ${LIBSYSTEMD_LIBRARIES})
         ELSE()
-          SET(LIBSYSTEMD ${LIBSYSTEMD_STATIC_LIBRARIES})
-          #SET(CMAKE_REQUIRED_FLAGS ${LIBSYSTEMD_STATIC_CFLAGS})
-          SET(MYSQLD_LINK_FLAGS "${MYSQLD_LINK_FLAGS} ${LIBSYSTEMD_STATIC_LDFLAGS}")
+          SET(LIBSYSTEMD ${LIBSYSTEMD_STATIC_LDFLAGS} ${LIBSYSTEMD_STATIC_LIBRARIES})
         ENDIF()
       ELSE()
         SET(LIBSYSTEMD systemd)