build: Remove AC_ISC_POSIX
The AC_ISC_POSIX macro is obsolete: https://www.gnu.org/software/autoconf/manual/autoconf-2.61/html_node/Posix-Variants.html
The AC_ISC_POSIX macro is obsolete: https://www.gnu.org/software/autoconf/manual/autoconf-2.61/html_node/Posix-Variants.html