[autoconf] reduce minimum automake version to 1.13
[autoconf] reduce minimum automake version to 1.13

Although removal of AM_PROG_CC_C_O in f107bac8 requires automake 1.14
to provide the same functionality in AC_PROG_CC, any widely used,
modern compiler supports cc -c -o.  Reducing the minimum required
automake version avoids the current need for Centos 7 maintainers
to patch configure.ac in order to build binary packages.

lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4