Updated ax_lua.m4 to current version from GNU Autoconf Archive's git
repository with additional patch to support OpenBSD.[1] Regenerated
``configure'' script. DOcumented changes.
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[1] https://savannah.gnu.org/patch/index.php?8355
use AC_SEARCH_LIB in configure.ac (shorter code)
replace '-lm -lcrypto -lz' by @LIBS@ in Makefile.in and AC_CHECK_LIB macros in configure.ac
replace NO_BACKTRACE by HAVE_EXECINFO_H