Postpone filter is an essential part of subrequest functionality.  In absence
of it a subrequest response body is sent to the client out of order with
respect to the main request header and body, as well as other subrequests.
For in-memory subrequests the response is also sent to the client instead of
being stored in memory.

Currently the postpone filter is automatically enabled if one of the following
standard modules which are known to create subrequests is enabled: ssi, slice,
addition.  However a third-party module that creates subrequests can still be
built without the postpone filter or be dynamically loaded in nginx built
without it.

By default, MSVC uses vc<version>.pdb in the current directory.
With the "-Fd" switch it is directed to be in the objs directory instead.

Unlike with GCC or MSVC, Clang linker doesn't auto-append ".exe" to the name
of the output binary when building on win32.

Since we now have both select and poll on Windows, it is enough to
do not set EVENT_FOUND, auto/modules will enable both automatically.

WSAPoll() is only available with Windows Vista and newer (and only
available during compilation if _WIN32_WINNT >= 0x0600).  To make
sure the code works with Windows XP, we do not redefine _WIN32_WINNT,
but instead load WSAPoll() dynamically if it is not available during
compilation.

Also, sockets are not guaranteed to be small integers on Windows.
So an index array is used instead of NGX_USE_FD_EVENT to map
events to connections.

Previously, select was compiled in by default, but the NGX_HAVE_SELECT
macro was not set, resulting in iocp being used by default unless
the "--with-select_module" configure option was explicitly specified.
Since the iocp module is not finished and does not work properly, this
effectively meant that the "--with-select_module" option was mandatory.

With the change NGX_HAVE_SELECT is properly set, making "--with-select_module"
optional.  Accordingly, it is removed from misc/GNUmakefile win32 target.

The module implements random load-balancing algorithm with optional second
choice.  In the latter case, the best of two servers is chosen, accounting
number of connections and server weight.

Example:

upstream u {
    random [two [least_conn]];

    server 127.0.0.1:8080;
    server 127.0.0.1:8081;
    server 127.0.0.1:8082;
    server 127.0.0.1:8083;
}

While 325b3042edd6 fixed it on MINIX, it broke it on systems
that output the word "version" on several lines with "cc -v".
The fix is to only consider "clang version" or "LLVM version"
as clang version, but this time only using sed(1).