BUILD: halog: fix a -Wundef warning on non-glibc systems

Dmitry reported this warning on FreeBSD since the introduction of -Wundef:

  admin/halog/fgets2.c:38:30: warning: '__GLIBC__' is not defined, evaluates to 0 [-Wundef]
  #if defined(__x86_64__) &&  (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 15))
                               ^
A defined() was missing.
1 file changed