Find non-default Homebrew installations in $HOMEBREW_ROOT

Fixes #788

Author: sodabrew

ext/mysql2/extconf.rb

@@ -52,6 +52,9 @@ def add_ssl_defines(header)
   /usr/local/opt/mysql5*
 ).map { |dir| dir << '/bin' }
 
+# For those without HOMEBREW_ROOT in PATH
+dirs << "#{ENV['HOMEBREW_ROOT']}/bin" if ENV['HOMEBREW_ROOT']
+
 GLOB = "{#{dirs.join(',')}}/{mysql_config,mysql_config5,mariadb_config}"
 
 # If the user has provided a --with-mysql-dir argument, we must respect it or fail.