[issue8510] update to autoconf2.65

[Matthias Klose]

Matthias Klose <doko at debian.org> added the comment:

r80965: Replace AC_HELP_STRING with AS_HELP_STRING
r80966: s/AC_AIX/AC_USE_SYSTEM_EXTENSIONS/
r80969: convert all obsolete AC_TRY_* macros to AC_*_IFELSE
r80970: Avoid autoconf warning: Assume C89 semantics that RETSIGTYPE is always void

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