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