Addon modules, both dynamic and static, can now use shared source files.
Shared sources result in only one make rule even if specified several
times in different modules.

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

Dependencies of dynamic modules are added to NGX_ADDON_DEPS (and
it is now used for dynamic modules) to be in line with what happens
in case of static compilation.

To avoid duplication, MAIL_DEPS and STREAM_DEPS are no longer passed
to auto/module when these modules are compiled as dynamic ones.  Mail
and stream dependencies are handled explicitly via corresponding
variables.

Previously flags passed by --with-ld-opt were not used when building perl
module, which meant hardening flags provided by package build systems were not
applied.

Just using "cp" is incorrect, as it will overwrite old files
possibly used by OS, leading to unexpected effects.  Changed
to "mv + cp", much like used for the main binary.

Removed extraneous braces around shell variables.

The "build" target introduced to do all build-related tasks, and
it is now used in Makefile and in objs/Makefile as a dependency for
the "install" target.

In particular, this resolves problems as observed with dynamic modules
by people trying to do "make install" without calling "make" first.