[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.

configure.ac

@@ -21,7 +21,7 @@ AC_CONFIG_MACRO_DIR([m4])
 
 AC_CANONICAL_HOST
 
-AM_INIT_AUTOMAKE([1.14 -Wall -Wno-portability -Wno-override foreign dist-xz tar-ustar serial-tests silent-rules])
+AM_INIT_AUTOMAKE([1.13 -Wall -Wno-portability -Wno-override foreign dist-xz tar-ustar serial-tests silent-rules])
 
 dnl @synopsis TRY_CFLAGS [compiler flags]
 dnl @summary check whether compiler supports given flags and adds them to CFLAGS