p11-kit's ./configure checks for secure_getenv()
AC_CHECK_FUNCS([getauxval issetugid getresuid secure_getenv])
However no matter whether it exists or not the p11-kit version in a compat.c is always used instead of the system one.
I /think/ that the only system providing secure_getenv() is glibc, however only if _GNU_SOURCE is set. Given that p11-kit contains code to prevent compilation with _GNU_SOURCE there does not seem to be any point in AC_CHECK_FUNCS([secure_getenv]) and modifying p11-kit's code to use the system version if available. How about dropping this check?