How do I make RbConfig aware of an include directory? (1.9.2 onFedora 16)

[Cactus]

I built 1.9.2-p320 from source on Fedora 16,
installed in /usr/local

I can't build the mysql2 gem because it thinks
that rb_thread_blocking_region isn't defined.
It is, but the conftest.c file that mkmf
generates fails because gcc can't find ruby.h

The reason why gcc can't find ruby.h is that the
gcc command-line generated from RbConfig doesn't
emit -I.... flags pointing to ruby.h and other
related .h files.

First, the .h files were installed in
/usr/local/include/ruby-1.9.1 although I was building
ruby 1.9.2 -- is this ok?

I could always add a symlink, but how do I make sure
that RbConfig knows about this location?

Thanks