compat: modify secure_getenv build error

In glibc 2.17, __secure_getenv was renamed to secure_getenv. So if we
use old glibc then "implicit declaration of function" error could be
occurred by "secure_getenv".
For compatibility, __secure_getenv is defined as secure_getenv in old
glibc.
https://sourceware.org/glibc/wiki/Tips_and_Tricks/secure_getenv

note: (As you may know) In systemd, missing.h support this. But, secure_getenv only is used at configurator.c now.