The warning seems questionable and overly strict.
For now, just disable it to allow building with gcc7.

    src/systemd/src/basic/time-util.c: In function ‘format_timespan’:
    src/systemd/src/basic/time-util.c:509:46: error: ‘%0*lu’ directive output between 1 and 2147483648 bytes may cause result to exceed ‘INT_MAX’ [-Werror=format-truncation=]
                                                  "%s"USEC_FMT".%0*"PRI_USEC"%s",
                                                  ^~~~
    src/systemd/src/basic/time-util.c:509:60: note: format string is defined here
                                                  "%s"USEC_FMT".%0*"PRI_USEC"%s",
    src/systemd/src/basic/time-util.c:509:46: note: directive argument in the range [0, 18446744073709551614]
                                                  "%s"USEC_FMT".%0*"PRI_USEC"%s",
                                                  ^~~~

https://mail.gnome.org/archives/networkmanager-list/2017-February/msg00001.html