Cannot Bundle Install due to an error compiling Unf_ext ' - ruby-on-rails

Question

I'm trying to install Huginn on a raspberry pi 3 running Debian Jesse (arm). Bundle install fails while trying to install unf_ext:
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
current directory: /tmp/bundler20161007-16062-unp7l7unf_ext-0.0.7.1/gems/unf_ext-0.0.7.1/ext/unf_ext
/usr/bin/ruby2.3 -r ./siteconf20161007-16062-wdgukt.rb extconf.rb
When I try: sudo gem install unf_ext -v '0.0.7.1' I get the following error:
current directory: /tmp/bundler20161007-16062-unp7l7unf_ext-0.0.7.1/gems/unf_ext-0.0.7.1/ext/unf_ext
make "DESTDIR="
compiling unf.cc
In file included from unf/normalizer.hh:10:0,
from unf.cc:1:
unf/table.hh:13539:25: error: narrowing conversion of ‘-27’ from ‘int’ to ‘char’ inside { } [-Wnarrowing]
-77, -39,-124, -39,-123};
The above is followed by a very long list of err: narrowing conversion of ## from int to char inside {} [-Wnarrowing] errors
I am compiling with: 6.1.1 20160802 (Raspbian 6.1.1-11+rpi1)
I tried gem update, but that didn't help. Nor did adding flags in ext/unf_ext/extconf.rb
+$CXXFLAGS += " -fsigned-char "
+
I'm a beginner with all of this, so any help would be greatly appreciated. Thanks.

Answer 1

NOTE: I've published my gem on Github to save people the effort of making their own. If you want to use mine, then just run these two commands:
wget https://github.com/appleorange1/unf_ext-0.0.6-arm/raw/master/unf_ext-0.0.6.gem
gem install ./unf_ext-0.0.6.gem
If you want to find out how I did this, keep reading:
I've was having this problem too (on a Chromebook C201PA). The issue is that your changes to ext/unf_ext/extconf.rb keep getting overwritten when you run "gem install". I'm not sure why this is, but I got around this problem by creating a local gem.
To do this, download the upstream gem version you want from this page. I used version 0.0.6.
Then, unpack the upstream gem:
gem unpack unf_ext-0.0.6.gem
After you've extracted the upstream gem, rename it so that you don't confuse it with your local gem. (You don't have to do this; it's just a suggestion.)
mv unf_ext-0.0.6.gem unf_ext-0.0.6-upstream.gem
Now, move into the unpacked gem.
cd unf_ext-0.0.6
Then, add the line $CXXFLAGS += ' -fsigned-char' to the file ext/unf_ext/extconf.rb. (I put it on the third line, just under have_library('stdc++').)
vi ext/unf_ext/extconf.rb
Then, you can build and install the gem:
gem build unf_ext.gemspec
gem install ./unf_ext-0.0.6.gem

Answer 2

I had a similar error with a later version bundled within a Rails project's Gemfile.lock:
bundle install
which called:
gem install unf_ext -v '0.0.7.2'
I was able to modify the Gemfile.lock and change the unf_ext version to 0.0.7.4; the unf_ext library has been updated in April 2017 to include ARM processor support. Read the GitHub issue here.
This is not as comprehensive as obscurityenthusiast's answer and could potentially break your solution. Do not change locked version strings without careful consideration.