This is to avoid setting the TCP_NODELAY flag on SPDY socket in
ngx_http_upstream_send_response().  The latter works per request,
but in SPDY case it might affect other streams in connection.

As of 1.3.9, chunked request body may be available with
r->headers_in.content_length_n <= 0.  Additionally, request body
may be in multiple buffers even if r->request_body_in_single_buf
was requested.

Dependancy tracking introduced in r5169 were not handled absolute path
names properly.  Absolute names might appear in CORE_DEPS if --with-openssl
or --with-pcre configure arguments are used to build OpenSSL/PCRE
libraries.

Additionally, revert part of r5169 to set NGX_INCS from Makefile
variables.  Makefile variables have $ngx_include_opt in them, which
might result in wrong include paths being used.  As a side effect,
this also restores build with --with-http_perl_module and --without-http
at the same time.

Before 1.3.9 an attempt to read body in a subrequest only caused problems
if body wasn't already read or discarded in a main request.  Starting with
1.3.9 it might also cause problems if body was discarded by a main request
before subrequest start.

Fix is to just ignore attempts to read request body in a subrequest, which
looks like right thing to do anyway.

Reported by Piotr Sikora.

To avoid further breaks it's now done properly, all the dependencies
are now passed to Makefile.PL.  While here, fixed include list passed to
Makefile.PL to use Makefile variables rather than a list expanded during
configure.

It will be called in ngx_http_upstream_finalize_request().