Add a dir_config for users on MacOS

- Users on MacOS that installed Ruby through different installer such
  as `ruby-install` will have their Ruby built with this
  `configure_args` `--with-libyaml-dir=/opt/homebrew/opt/libyaml`.

  This commit adds a `dir_config("libyaml")` so that the flag is taken
  in consideration. Otherwise the header and function can't be found
  leading to a compilation error.

  I'm not super familiar with C extensions, but from reading the doc
  it seems that `dir_config` just "pushes" to the list of directories
  ruby will search for libraries and include files. It shouldn't
  affect users that are not on MacOS.

Author: Edouard-chin

system/README.md

@@ -45,11 +45,14 @@ That's pretty simple, but is enough to demonstrate the integration with `libyaml
 The `extconf.rb` is still simple (and similar to `isolated/ext/isolated/extconf.rb` but contains this additional block:
 
 ``` ruby
+dir_config('libyaml')
 unless find_header("yaml.h") && find_library("yaml", "yaml_get_version")
   abort("\nERROR: *** could not find libyaml development environment ***\n\n")
 end
 ```
 
+`dir_config` is optional and mostly for users on MacOS as most ruby installers build Ruby with the `--with-libyaml-dir=/opt/homebrew/opt/libyaml` flag. The `dir_config` allows for this flag to be taken in consideration and help Ruby determine where to search for the yaml library.
+
 `find_header` and `find_library` are `MakeMakefile` helper methods which will search your system's standard directories looking for files. If it finds them, it makes sure the compile step will be able to find `yaml.h`, and the link step will be able to find the `libyaml` library file.
 
 We ask `find_header` to look for the `yaml.h` header file because that's what our C code needs (see `ext/system/system.h`). We ask `find_library` to look for a library named `libyaml` and check that it has the function `yaml_get_version()` defined in it.

system/ext/system/extconf.rb

@@ -1,5 +1,6 @@
 require "mkmf"
 
+dir_config('libyaml')
 unless find_header("yaml.h") && find_library("yaml", "yaml_get_version")
   abort("\nERROR: *** could not find libyaml development environment ***\n\n")
 end