MacOS: Add a Homebrew OpenSSL directory configuration option.

This commit is to enable users to set a custom Homebrew openssl directory
configuration option to set the openssl library path on MacOS.

Previously we got the openssl library path by `brew --prefix openssl` on MacOS.
However, there is a case where the directory actually doesn't exist. And this
option enables users to build mysql2 in the case.

```
+ brew --prefix openssl
/usr/local/opt/openssl@3

+ ls -ld /usr/local/opt/openssl*
lrwxr-xr-x  1 runner  admin  36 Dec 16 01:28 /usr/local/opt/openssl -> /usr/local/Cellar/[email protected]/1.1.1s
lrwxr-xr-x  1 runner  admin  28 Dec 16 01:19 /usr/local/opt/[email protected] -> ../Cellar/[email protected]/1.1.1s
/usr/local/opt/[email protected]
```

== How to use ==

In the development, you can run like this.

```
$ RUBY_MYSQL2_BREW_SSL_DIR=/usr/local/opt/[email protected] rake
```

In the installation, you can run like this.

```
$ gem install mysql2 -- --with-brew-ssl-dir=/usr/local/opt/[email protected]
```

== Note ==

In this commit, the directory existing check is added in the `extconf.rb`. If
the openssl directory doesn't exist, the build fails with the error message
below.

```
Cannot find library dir(s) /usr/local/opt/openssl@3/lib
```

.github/workflows/build.yml

@@ -1,5 +1,10 @@
 name: Build
 on: [push, pull_request]
+env:
+  BUNDLE_WITHOUT: development
+  # Reduce MacOS CI time, don't need to clean a runtime that isn't saved
+  HOMEBREW_NO_INSTALL_CLEANUP: 1
+  HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: 1
 jobs:
   build:
     name: >-
@@ -27,30 +32,37 @@ jobs:
           - '2.2'
           - '2.1'
         db: ['']
+        env: [{}]
         include:
           # Comment out due to ci/setup.sh stucking.
           # - {os: ubuntu-18.04, ruby: 2.4, db: mariadb10.1}
-          - {os: ubuntu-20.04, ruby: '2.4', db: mariadb10.3}
-          - {os: ubuntu-18.04, ruby: '2.4', db: mysql57}
-          - {os: ubuntu-20.04, ruby: '2.4', db: mysql80}
-          - {os: ubuntu-18.04, ruby: 'head', db: ''}
+          - {os: ubuntu-20.04, ruby: '2.4', db: mariadb10.3, env: {}}
+          - {os: ubuntu-18.04, ruby: '2.4', db: mysql57, env: {}}
+          - {os: ubuntu-20.04, ruby: '2.4', db: mysql80, env: {}}
+          - {os: ubuntu-18.04, ruby: 'head', db: '', env: {}}
           # db: A DB's brew package name in macOS case.
           #     Set a name "db: '[email protected]'" when using an old version.
           # MariaDB lastet version
           # Allow failure due to the following test failures that rarely happens.
           # https://github.com/brianmario/mysql2/issues/1194
-          - {os: macos-latest, ruby: '2.6', db: mariadb, allow-failure: true}
+          - os: macos-latest
+            ruby: '2.6'
+            db: mariadb
+            allow-failure: true
+            env:
+              RUBY_MYSQL2_HOMEBREW_SSL_DIR: '/usr/local/opt/[email protected]'
           # MySQL latest version
           # Allow failure due to the issue #1194.
-          - {os: macos-latest, ruby: '2.6', db: mysql, allow-failure: true}
+          - os: macos-latest
+            ruby: '2.6'
+            db: mysql
+            allow-failure: true
+            env:
+              RUBY_MYSQL2_HOMEBREW_SSL_DIR: '/usr/local/opt/[email protected]'
       # On the fail-fast: true, it cancels all in-progress jobs
       # if any matrix job fails unlike Travis fast_finish.
       fail-fast: false
-    env:
-      BUNDLE_WITHOUT: development
-      # reduce MacOS CI time, don't need to clean a runtime that isn't saved
-      HOMEBREW_NO_INSTALL_CLEANUP: 1
-      HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: 1
+    env: ${{ matrix.env }}
     steps:
       - uses: actions/checkout@v3
       # https://github.com/ruby/setup-ruby

ext/mysql2/extconf.rb

@@ -28,8 +28,16 @@ def add_ssl_defines(header)
 
 # Homebrew openssl
 if RUBY_PLATFORM =~ /darwin/ && system("command -v brew")
-  openssl_location = `brew --prefix openssl`.strip
-  $LDFLAGS << " -L#{openssl_location}/lib" if openssl_location
+  _, lib = dir_config('brew-ssl')
+  unless lib
+    openssl_location = `brew --prefix openssl`.strip
+    lib = "#{openssl_location}/lib" if openssl_location
+  end
+
+  if lib
+    abort "-----\nCannot find library dir(s) #{lib}\n-----" unless lib && lib.split(File::PATH_SEPARATOR).any? { |dir| File.directory?(dir) }
+    $LDFLAGS << " -L#{lib}"
+  end
 end
 
 # 2.1+

tasks/compile.rake

@@ -51,6 +51,9 @@ Rake::ExtensionTask.new("mysql2", Mysql2::GEMSPEC) do |ext|
       POST_INSTALL_MESSAGE
     end
   end
+
+  brew_ssl_dir = ENV['RUBY_MYSQL2_HOMEBREW_SSL_DIR']
+  ext.config_options << "--with-brew-ssl-dir=#{brew_ssl_dir}" if brew_ssl_dir
 end
 Rake::Task[:spec].prerequisites << :compile