mkmf and environment variables

[has]

Hi all,

I need to set an environment variable before building a Ruby C
extension:

MACOSX_DEPLOYMENT_TARGET=10.2
export MACOSX_DEPLOYMENT_TARGET

For simplicity and robustness I'd rather this happened automatically
when the Makefile was run. Is there some way for by extconf.rb file to
tell mkmf to add the above lines to the Makefile when generating it?

Many thanks,

has