Add -fdeclspec to NXT_RUBY_CFLAGS for Clang, if it's available.

Clang incorrectly reports 1 for __has_declspec_attribute(x) in
some cases, such as MacOS or Cygwin.  That causes ruby code to
break.  ruby added -fdeclspec to their CFLAGS in 2019 to
workaround this bug, since it enables __declspec() and therefore,
the compiler behavior matches what it reports.

Since we don't know what are all the architectures that trigger
the clang bug, let's add the flag for all of them (especially
since it should be harmless).

Add this workaround only at the time of configuring the ruby
module.  This way we don't clutter the global NXT_CFLAGS with an
unnecessary flag.

Link: unit bug <https://github.com/nginx/unit/issues/653>
Link: ruby bug <https://bugs.ruby-lang.org/issues/18616>
Link: LLVM bug <https://github.com/llvm/llvm-project/issues/49958>
Commit: LLVM: Add -fdeclspec <d170c4b57a91adc74ca89c6d4af616a00323b12c>
Commit: ruby: Use -fdeclspec <0958e19ffb047781fe1506760c7cbd8d7fe74e57>

Now it's possible to disable default bind mounts of
languages by setting:

  {
      "isolation": {
           "automount": {
               "language_deps": false
           }
     }
  }

In this case, the user is responsible to provide a "rootfs"
containing the language libraries and required files for
the application.

The gem paths must depend on the specified interpreter.
Also, gemdir looks redundant as it's already included in Gem.default_path().

There is no reason to use printf instead of just print.

No functional changes.

An attempt to build a Ruby module for a custom Ruby installation that has the
same major version as the system Ruby may unexpectedly cause the use of the
system Ruby library.

This closes #449 issue on GitHub.

Since the introduction of rootfs feature, some language modules
can't be configured multiple times.

Now the configure generates a separate nxt_<module>_mounts.h for
each module compiled.

Currently almost all Unit object files depends on generated nxt_version.h.
This patch adds missing dependence and fixes running make with multiple
jobs.

This closes #318 issue on GitHub.