build: s/AC_CONFIG_MACRO_DIRS/AC_CONFIG_MACRO_DIR/

Apparently AC_CONFIG_MACRO_DIRS exists first since 2013, but
AC_CONFIG_MACRO_DIR has existed far longer. This fixes auto(re)conf on
systems with pre-1.13 automake (pre-2.70 autoconf).

Reported by @kvishnivetsky, closes #113.