$ gem install cool.io -v 1.7.1
Building native extensions. This could take a while...
ERROR:  Error installing cool.io:
        ERROR: Failed to build gem native extension.

    current directory: /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/gems/cool.io-1.7.1/ext/cool.io
/home/watson/.rbenv/versions/3.4.9/bin/ruby extconf.rb
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_without_gvl()... yes
checking for rb_thread_alone()... yes
checking for rb_str_set_len()... yes
checking for clock_gettime() in -lrt... yes
checking for ruby/io.h... yes
checking for ruby/thread.h... yes
checking for sys/select.h... yes
checking for poll.h... yes
checking for sys/epoll.h... yes
checking for sys/event.h... no
checking for port.h... no
checking for sys/resource.h... yes
creating Makefile

current directory: /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/gems/cool.io-1.7.1/ext/cool.io
make DESTDIR\= sitearchdir\=./.gem.20260322-9862-z2rih5 sitelibdir\=./.gem.20260322-9862-z2rih5 clean

current directory: /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/gems/cool.io-1.7.1/ext/cool.io
make DESTDIR\= sitearchdir\=./.gem.20260322-9862-z2rih5 sitelibdir\=./.gem.20260322-9862-z2rih5
compiling cool.io_ext.c
In file included from cool.io_ext.c:11:
cool.io.h:21:23: warning: 'RUBY_VERSION_MAJOR' is not defined, evaluates to '0' [-Wundef]
   21 | #if !HAVE_RB_IO_T || (RUBY_VERSION_MAJOR == 1 && RUBY_VERSION_MINOR == 8)
      |                       ^~~~~~~~~~~~~~~~~~
cool.io_ext.c:13:14: warning: 'mCoolio' defined but not used [-Wunused-variable]
   13 | static VALUE mCoolio = Qnil;
      |              ^~~~~~~
cc1: note: unrecognized command-line option '-Wno-self-assign' may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option '-Wno-parentheses-equality' may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option '-Wno-constant-logical-operand' may have been intended to silence earlier diagnostics
compiling iowatcher.c

(...snip...)

../libev/ev.h:738:79: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  738 | # define ev_set_cb(ev,cb_)                   (ev_cb_ (ev) = (cb_), memmove (&((ev_watcher *)(ev))->cb, &ev_cb_ (ev), sizeof (ev_cb_ (ev))))
      |                                                                              ~^~~~~~~~~~~~~~~~~~~
../libev/ev.h:690:3: note: in expansion of macro 'ev_set_cb'
  690 |   ev_set_cb ((ev), cb_);                        \
      |   ^~~~~~~~~
../libev/ev.c:5103:3: note: in expansion of macro 'ev_init'
 5103 |   ev_init (&once->io, once_cb_io);
      |   ^~~~~~~
../libev/ev.h:687:4: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  687 |   ((ev_watcher *)(void *)(ev))->active  =       \
      |   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../libev/ev.c:5110:3: note: in expansion of macro 'ev_init'
 5110 |   ev_init (&once->to, once_cb_to);
      |   ^~~~~~~
../libev/ev.h:688:4: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  688 |   ((ev_watcher *)(void *)(ev))->pending = 0;    \
      |   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../libev/ev.c:5110:3: note: in expansion of macro 'ev_init'
 5110 |   ev_init (&once->to, once_cb_to);
      |   ^~~~~~~
../libev/ev.h:732:50: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  732 | # define ev_set_priority(ev,pri)             (   (ev_watcher *)(void *)(ev))->priority = (pri)
      |                                              ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../libev/ev.h:689:3: note: in expansion of macro 'ev_set_priority'
  689 |   ev_set_priority ((ev), 0);                    \
      |   ^~~~~~~~~~~~~~~
../libev/ev.c:5110:3: note: in expansion of macro 'ev_init'
 5110 |   ev_init (&once->to, once_cb_to);
      |   ^~~~~~~
../libev/ev.h:738:79: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  738 | # define ev_set_cb(ev,cb_)                   (ev_cb_ (ev) = (cb_), memmove (&((ev_watcher *)(ev))->cb, &ev_cb_ (ev), sizeof (ev_cb_ (ev))))
      |                                                                              ~^~~~~~~~~~~~~~~~~~~
../libev/ev.h:690:3: note: in expansion of macro 'ev_set_cb'
  690 |   ev_set_cb ((ev), cb_);                        \
      |   ^~~~~~~~~
../libev/ev.c:5110:3: note: in expansion of macro 'ev_init'
 5110 |   ev_init (&once->to, once_cb_to);
      |   ^~~~~~~
../libev/ev.h:694:52: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  694 | #define ev_timer_set(ev,after_,repeat_)      do { ((ev_watcher_time *)(ev))->at = (after_); (ev)->repeat = (repeat_); } while (0)
      |                                                   ~^~~~~~~~~~~~~~~~~~~~~~~~
../libev/ev.c:5113:7: note: in expansion of macro 'ev_timer_set'
 5113 |       ev_timer_set (&once->to, timeout, 0.);
      |       ^~~~~~~~~~~~
../libev/ev.c: At top level:
../libev/ev.c:5230:27: warning: '/*' within comment [-Wcomment]
 5230 | /* EV_STAT     0x00001000 /* stat data changed */
../libev/ev.c:5231:27: warning: '/*' within comment [-Wcomment]
 5231 | /* EV_EMBED    0x00010000 /* embedded event loop needs sweep */
cc1: note: unrecognized command-line option '-Wno-self-assign' may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option '-Wno-parentheses-equality' may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option '-Wno-constant-logical-operand' may have been intended to silence earlier diagnostics
make: *** [Makefile:251: libev.o] Error 1

make failed, exit code 2

Gem files will remain installed in /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/gems/cool.io-1.7.1 for inspection.
Results logged to /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/extensions/x86_64-linux/3.4.0/cool.io-1.7.1/gem_make.out