Ship precompiled gems for linux

Hello! If you're arriving here to give feedback on the v1.11.0 Release Candidates, please make sure to:

tell us which linux distro you're using!
(If you can) a docker image in which we can reproduce the failure you're seeing!

Thanks so much for using Nokogiri and for giving us feedback on this experiment!

See #1571 for the PR from @larskanis that enables building precompiled libraries for linux.

This issue is for discussion on how to ship those precompiled libraries in a gem for linux users in a way that won't break the installation experience for people.

Some things that we'll need to figure out were mentioned in #1571 by @larskanis:

Bundler isn't clever at selection of unsupported ruby versions. See Add two specs for choosing platform specific gems rubygems/bundler#6247 . This requires users to explicit opt-out of binary gems, when they want to run nokogiri on ruby-trunk.

There is a known issue on Alpine Linux: Cross building doesn't work for alpine3.7 rake-compiler/rake-compiler-dock#20 . I did a manual test on the alpine docker image and it reproducible fails at Nokogiri::XML::Reader#namespaces somewhere within the tests: https://gist.github.com/larskanis/94920901e6466c9383360ee1346f9f34 . I didn't analyze the reason for the segfault so far, but I think we should before the release.

And he later mentioned:

Issue 1 in the above comment will probably finally be fixed by the next bundler release. See rubygems/bundler#7522 . This way bundler will select the platform=ruby gem automatically, if fat binary gem doesn't fit to the ruby release.

I think we'll probably also be able to fix / work around issue 2, so that I hope we'll be able to ship binary releases some day.

Finally, I'd like for VersionInfo to contain some indication that the user is using precompiled libraries. A patch that worked at one point on the #1571 branch (but now needs to be rewritten) is:

From 20f69908732b50258be0ee18e1d78b841d462dc4 Mon Sep 17 00:00:00 2001
From: Mike Dalessio <mike.dalessio@gmail.com>
Date: Sun, 13 Jan 2019 01:15:06 -0500
Subject: [PATCH] `nokogiri -v` emits info about precompiled binaries

It may be useful information to be able to tell if the libraries were
compiled on installation, or whether they were precompiled in a fat
binary native gem.
---
 Rakefile                | 3 +++
 ext/nokogiri/nokogiri.c | 6 ++++++
 lib/nokogiri/version.rb | 6 ++++++
 3 files changed, 15 insertions(+)

diff --git a/Rakefile b/Rakefile
index 5ef937d..72edb7e 100644
--- a/Rakefile
+++ b/Rakefile
@@ -383,6 +383,9 @@ task :cross do
     raise "rake-compiler has not installed any cross rubies. Use rake-compiler-dock or 'rake gem:windows' for building binary windows gems."
   end
 
+  ENV['CFLAGS'] ||= ""
+  ENV['CFLAGS'] += " -DNOKOGIRI_USE_PRECOMPILED_NATIVE"
+
   CROSS_RUBIES.each do |cross_ruby|
     task "tmp/#{cross_ruby.platform}/nokogiri/#{cross_ruby.ver}/nokogiri.so" do |t|
       # To reduce the gem file size strip mingw32 dlls before packaging
diff --git a/ext/nokogiri/nokogiri.c b/ext/nokogiri/nokogiri.c
index 740b30d..5b32500 100644
--- a/ext/nokogiri/nokogiri.c
+++ b/ext/nokogiri/nokogiri.c
@@ -104,6 +104,12 @@ void Init_nokogiri()
   rb_const_set(mNokogiri, rb_intern("NOKOGIRI_LIBXSLT_PATCHES"), Qnil);
 #endif
 
+#ifdef NOKOGIRI_USE_PRECOMPILED_NATIVE
+  rb_const_set(mNokogiri, rb_intern("NOKOGIRI_USE_PRECOMPILED_NATIVE"), Qtrue);
+#else
+  rb_const_set(mNokogiri, rb_intern("NOKOGIRI_USE_PRECOMPILED_NATIVE"), Qfalse);
+#endif
+
 #ifdef LIBXML_ICONV_ENABLED
   rb_const_set(mNokogiri, rb_intern("LIBXML_ICONV_ENABLED"), Qtrue);
 #else
diff --git a/lib/nokogiri/version.rb b/lib/nokogiri/version.rb
index 44c076a..c05e1a0 100644
--- a/lib/nokogiri/version.rb
+++ b/lib/nokogiri/version.rb
@@ -46,6 +46,10 @@ module Nokogiri
       NOKOGIRI_USE_PACKAGED_LIBRARIES
     end
 
+    def libxml2_using_precompiled_native?
+      NOKOGIRI_USE_PRECOMPILED_NATIVE
+    end
+
     def warnings
       warnings = []
 
@@ -78,6 +82,7 @@ module Nokogiri
           vi["libxml"] = {}.tap do |libxml|
             if libxml2_using_packaged?
               libxml["source"] = "packaged"
+              libxml["precompiled"] = libxml2_using_precompiled_native?
               libxml["patches"] = NOKOGIRI_LIBXML2_PATCHES
             else
               libxml["source"] = "system"
@@ -89,6 +94,7 @@ module Nokogiri
           vi["libxslt"] = {}.tap do |libxslt|
             if libxml2_using_packaged?
               libxslt["source"] = "packaged"
+              libxslt["precompiled"] = libxml2_using_precompiled_native?
               libxslt["patches"] = NOKOGIRI_LIBXSLT_PATCHES
             else
               libxslt["source"] = "system"
-- 
2.17.1

And tagging @tenderlove for visibility because he had some thoughts on the previous #1571 thread.