Add check for the presence of secure_getenv

[irozzo-1A]

This change introduces support for glibc versions older than 2.17 where secure_getenv was introduced.

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[brendandburns]

This looks good to me. I will wait for @ityuhui to have a look also before we merge.

Thanks for the PR!

[ityuhui]

It's good to me too.
But I think there is a small problem in the previous code (even though it has no effect on the functionality):

config.h.in

/* Define to 1 if you have the `strndup' function. */
#cmakedefine HAVE_STRNDUP 1
#cmakedefine HAVE_SECURE_GETENV 1

The code should change to

#cmakedefine HAVE_STRNDUP
#cmakedefine HAVE_SECURE_GETENV

because the HAVE_STRNDUP and HAVE_SECURE_GETENV are determined by
kubernetes/ConfigureChecks.cmake

check_function_exists(strndup HAVE_STRNDUP)
check_function_exists(secure_getenv HAVE_SECURE_GETENV)

I'll submit another PR to update it.

[ityuhui]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: irozzo-1A, ityuhui

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[ityuhui]

Thanks for this PR !