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.

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's no longer necessary to update src/http/modules/perl/nginx.pm
when version is bumped, as it's now derived from src/core/nginx.h.

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.

Added missing dependencies for perl module's Makefile.

Simplified dependencies for perl module nginx.so: it depends
on Makefile that in turn depends on other perl bits.

No functional changes.

This allows to automatically pick the preferred system compiler on
systems with multiple compilers.

Reviewed by:	mdounin, ru

Note: the "-p" argument of cp(1) dropped intentionally, to force nginx.so
rebuild.  It is considered too boring to properly list all dependencies
in Makefile.PL.