Filename extension used for dynamically loaded perl modules isn't
necessarily ".so" (e.g., it's ".bundle" on Mac OS X).

This fixes "make" after "make" unnecessarily rebuilding perl module.

Now perl configure will correctly fail if ExtUtils::Embed perl module
is not present in the system (found on Amazon Linux AMI, as of
release 2012.03).

unsupported threaded environment, but now they complicate code:
*) perl_clone() requires at least duplicating nginx stash;
*) the multiplicity requires to re-evalute all precompiled subroutines
   and nginx stash in new interpreter context.

    *) Change in internal API: the HTTP modules initialization was moved
       from the init module phase to the HTTP postconfiguration phase.

    *) Change: now the request body is not read beforehand for the
       ngx_http_perl_module: it's required to start the reading using the
       $r->has_request_body method.

    *) Feature: the ngx_http_perl_module supports the DECLINED return code.

    *) Feature: the ngx_http_dav_module supports the incoming "Date" header
       line for the PUT method.

    *) Feature: the "ssi" directive is available inside the "if" block.

    *) Bugfix: a segmentation fault occurred if there was an "index"
       directive with variables and the first index name was without
       variables; the bug had appeared in 0.1.29.

    *) Feature: the "dav_access" directive.

    *) Feature: the "if" directive supports the "-d", "!-d", "-e", "!-e",
       "-x", and "!-x" operators.

    *) Bugfix: a segmentation fault occurred if a request returned a
       redirect and some sent to client header lines were logged in the
       access log.

    *) Feature: the ngx_http_perl_module.

    *) Change: the "valid_referers" directive allows the referreres without
       URI part.