Dynamically set Homebrew-installed OpenSSL flag (#1204)

This is a follow-up to #1135 which added the OpenSSL flag assuming that
if the `RUBY_PLATFORM` is `darwin` (macOS):
- Homebrew is installed
- OpenSSL is installed via Homebrew

This PR:
- no longer assumes Homebrew is installed if we're on macOS
- no longer assumes OpenSSL is installed via Homebrew
- asks Homebrew for the openssl location (which will also work with the
newer openssl@1.1 recipe)

Should prevent issues like these when running bundle install on the
rails codebase:

    Bundle Install error due to: ld: warning: directory not found for option '-L/usr/local/opt/openssl/lib'

Author: olivierlacan

ext/mysql2/extconf.rb

@@ -22,7 +22,12 @@ def add_ssl_defines(header)
 end
 
 # Homebrew openssl
-$LDFLAGS << ' -L/usr/local/opt/openssl/lib' if RUBY_PLATFORM =~ /darwin/
+if RUBY_PLATFORM =~ /darwin/ && system("command -v brew")
+  openssl_location = `brew --prefix openssl`.strip
+  if openssl_location
+    $LDFLAGS << " -L#{openssl_location}/lib" 
+  end
+end
 
 # 2.1+
 have_func('rb_absint_size')