configure: initialize automake with subdir-objects option

This makes the buildsystem forward compatible with automake>=1.14 since
nghttp uses a recursive build. It avoids a large and non-scary warning
on running autoreconf, summarized as:

src/Makefile.am:57: warning: source file 'http-parser/http_parser.c' is in a subdirectory,
src/Makefile.am:57: but option 'subdir-objects' is disabled

configure.ac

@@ -44,7 +44,7 @@ AC_CANONICAL_TARGET
 
 AC_CONFIG_MACRO_DIR([m4])
 
-AM_INIT_AUTOMAKE()
+AM_INIT_AUTOMAKE([subdir-objects])
 AC_CONFIG_HEADERS([config.h])
 
 dnl Checks for command-line options