Gem build options -- how to make them persist?

On one of my systems, the pg gem won’t build unless you tell it the
non-standard header location:

gem install pg – --with-pg-dir=/usr/pgsql-9.0

gem update pg – --with-pg-dir=/usr/pgsql-9.0

Now, how can I set this option once and for all, so that gem update
always uses it?

I tried putting this in my .gemrc:

gem: --with-pg-dir=/usr/pgsql-9.0

but that doesn’t work (as expected, since those options are passed to
gem rather than the gem’s extconf).

There is no way. I have a ticket filed for this feature. Patches

If you’re using bundler, bundle config can do this for you.