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>

The patch removes the "files" section from package.json to avoid future issues
with missing files.  For package testing purposes, 'npm pack' is used instead
of plain 'tar' to simulate packaging more accurately.

In Go 1.16, the module-aware mode is enabled by default; to fall back to
previous behavior, the GO111MODULE environment variable should be set to
'auto'.

Details: https://golang.org/doc/go1.16

This makes the "sed" instruction simpler and more portable, as the previous
variant didn't work well on BSD systems due to the "\s" metacharacter.

Thanks to Sergey A. Osokin <osa@FreeBSD.org.ru> for spotting this issue.

Also, this should prevent accidentally creating a version 1.0.0 package.

Globally installed modules require a globally installed libunit.
The "binding_pub.gyp" file is the correct version, otherwise linked module
may have unresolved symbols because libunit is not linked.

Thanks to @wujjpp.

This closes #490 PR on GitHub.