compat: address "old-style-definition" compiler error on Ruby 3.1+

[dmke]

This fixes an issue with Ruby 3.1+, where the installation fails on compiling the extension.

Compare the following outputs of rake compile of various Ruby versions:

Ruby 3.0.7

https://github.com/digineo/visibility_monitor/actions/runs/8877708931/job/24371823796

/opt/hostedtoolcache/Ruby/3.0.7/x64/bin/ruby -I. ../../../../ext/visibility_monitor_c/extconf.rb
mkdir -p tmp/x86_64-linux/visibility_monitor_c/3.0.7
cd tmp/x86_64-linux/visibility_monitor_c/3.0.7
creating Makefile
/usr/bin/gmake
cd -
cd tmp/x86_64-linux/visibility_monitor_c/3.0.7
compiling ../../../../ext/visibility_monitor_c/ext.c
linking shared-object visibility_monitor_c.so
/usr/bin/gmake install sitearchdir=../../../../lib sitelibdir=../../../../lib target_prefix=
cd -
mkdir -p tmp/x86_64-linux/stage/lib
/usr/bin/install -c -m 0755 visibility_monitor_c.so ../../../../lib
cp tmp/x86_64-linux/visibility_monitor_c/3.0.7/visibility_monitor_c.so tmp/x86_64-linux/stage/lib/visibility_monitor_c.so

Ruby 3.1.5

https://github.com/digineo/visibility_monitor/actions/runs/8877708931/job/24371824086

/opt/hostedtoolcache/Ruby/3.1.5/x64/bin/ruby -I. ../../../../ext/visibility_monitor_c/extconf.rb
mkdir -p tmp/x86_64-linux/visibility_monitor_c/3.1.5
cd tmp/x86_64-linux/visibility_monitor_c/3.1.5
creating Makefile
/usr/bin/gmake
cd -
cd tmp/x86_64-linux/visibility_monitor_c/3.1.5
compiling ../../../../ext/visibility_monitor_c/ext.c
../../../../ext/visibility_monitor_c/ext.c: In function ‘Init_visibility_monitor_c’:
../../../../ext/visibility_monitor_c/ext.c:39:6: error: old-style function definition [-Werror=old-style-definition]
   39 | void Init_visibility_monitor_c() {
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~
../../../../ext/visibility_monitor_c/ext.c: At top level:
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
cc1: all warnings being treated as errors
gmake: *** [Makefile:247: ext.o] Error 1
rake aborted!
Command failed with status (2): [/usr/bin/gmake]
/home/runner/work/visibility_monitor/visibility_monitor/vendor/bundle/ruby/3.1.0/gems/rake-compiler-1.2.7/lib/rake/extensiontask.rb:195:in `block (2 levels) in define_compile_tasks'
/home/runner/work/visibility_monitor/visibility_monitor/vendor/bundle/ruby/3.1.0/gems/rake-compiler-1.2.7/lib/rake/extensiontask.rb:194:in `block in define_compile_tasks'
/home/runner/work/visibility_monitor/visibility_monitor/vendor/bundle/ruby/3.1.0/gems/rake-13.2.1/exe/rake:27:in `<top (required)>'
/opt/hostedtoolcache/Ruby/3.1.5/x64/bin/bundle:25:in `load'
/opt/hostedtoolcache/Ruby/3.1.5/x64/bin/bundle:25:in `<main>'
Tasks: TOP => compile => compile:x86_64-linux => compile:visibility_monitor_c:x86_64-linux => copy:visibility_monitor_c:x86_64-linux:3.1.5 => tmp/x86_64-linux/visibility_monitor_c/3.1.5/visibility_monitor_c.so
(See full trace by running task with --trace)

Ruby 3.2.4

https://github.com/digineo/visibility_monitor/actions/runs/8877708931/job/24371824399

Output similar to Ruby 3.1.

Ruby 3.3.1

https://github.com/digineo/visibility_monitor/actions/runs/8877708931/job/24371824707

Output similar to Ruby 3.1.

The core of this PR is the last commit (HEAD):

diff --git a/ext/visibility_monitor_c/ext.c b/ext/visibility_monitor_c/ext.c
index 3964e7b..7a95a87 100644
--- a/ext/visibility_monitor_c/ext.c
+++ b/ext/visibility_monitor_c/ext.c
@@ -36,7 +36,7 @@ static VALUE monitor_set_visibility_placeholder(VALUE mod, VALUE method_name, VA
     return Qnil;
 }
 
-void Init_visibility_monitor_c() {
+void Init_visibility_monitor_c(void) {
     VALUE mVisibilityMonitor = rb_define_module("VisibilityMonitor");
 
     rb_define_method(mVisibilityMonitor, "public", monitor_public, -1);

You can ignore the remaining commits, although they've proven useful for further development:

• HEAD~1 relaxes dev-dependency version constraints (basically to allow a more modern Bundler version)
• HEAD~2 removes Travis CI and introduces Github Actions