title: gem install mailcatcher fails with - ERROR: Error installing mailcatcher: ERROR: Failed to build gem native extension
body: <p>I am on macOS Catalina <code>10.15.7</code>, using rbenv (version <code>rbenv 1.1.2-36-g60c9339</code> ) and ruby verion <code>2.6.3</code> (<code>ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin19]</code> is the output of <code>ruby -v</code>) for my project.
  I need to install mailcatcher to check the emails my application generates.
  On running <code>gem install mailcatcher</code> in my project folder, the command fails with the following error:</p>
  <pre><code>➜ gem install mailcatcher
  Building native extensions. This could take a while...
  ERROR:  Error installing mailcatcher:
      ERROR: Failed to build gem native extension.
  
      current directory: /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-1.5.1/ext/thin_parser
  /Users/karansapolia/.rbenv/versions/2.6.3/bin/ruby -I /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/2.6.0 -r ./siteconf20201103-23665-1tzqkaj.rb extconf.rb
  checking for main() in -lc... yes
  creating Makefile
  
  current directory: /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-1.5.1/ext/thin_parser
  make &quot;DESTDIR=&quot; clean
  
  current directory: /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-1.5.1/ext/thin_parser
  make &quot;DESTDIR=&quot;
  compiling parser.c
  parser.rl:112:17: warning: comparison of integers of different signs: 'long' and 'unsigned long' [-Wsign-compare]
    assert(pe - p == len - off &amp;&amp; &quot;pointers aren't same distance&quot;);
           ~~~~~~ ^  ~~~~~~~~~
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/assert.h:93:25: note: expanded from macro 'assert'
      (__builtin_expect(!(e), 0) ? __assert_rtn(__func__, __FILE__, __LINE__, #e) : (void)0)
                          ^
  parser.rl:142:7: error: implicit declaration of function 'thin_http_parser_has_error' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    if (thin_http_parser_has_error(parser) ) {
        ^
  parser.rl:142:7: note: did you mean 'http_parser_has_error'?
  ./parser.h:44:5: note: 'http_parser_has_error' declared here
  int http_parser_has_error(http_parser *parser);
      ^
  parser.rl:144:14: error: implicit declaration of function 'thin_http_parser_is_finished' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    } else if (thin_http_parser_is_finished(parser) ) {
               ^
  1 warning and 2 errors generated.
  make: *** [parser.o] Error 1
  
  make failed, exit code 2
  
  Gem files will remain installed in /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-1.5.1 for inspection.
  Results logged to /Users/karansapolia/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/thin-1.5.1/gem_make.out
  </code></pre>
  <p>Installing other gems is working perfectly. <code>gem install mailman</code> worked perfectly. Only fails for <code>mailcatcher</code>.</p>
  <p>How do I install mailcatcher without the errors?</p>