Fix autotools where ax_config_feature and ax_have_epoll do not exist.

Reported and tested by @vodik, closes #110.
Merge branch 'fix/autotools-110'
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.