Solve install problem when using PostgresApp Mac OS X

Are you using PostgresApp and trying to install PostgreSQL gem and returning Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension. ?

I did this on Mavericks and worked. This can maybe happen on other versions of OS X.

gem install pg -- --with-pg-config="/Applications/Postgres.app/Contents/MacOS/bin/pg_config"

After do this I could bundle normally.