I want to install a gem but I get the following error:
Building native extensions. This could take a while...
ERROR: Error installing twitter-login:
ERROR: Failed to build gem native extension.
NotImplementedError: C extension support is not enabled. Pass -Xcext.enabled=true to JRuby or set JRUBY_OPTS or modify .jrubyrc to enable.
.jrubyrc edits suggested by Rodrigo didn't work for me (jdk 7u25 / JRuby 1.7.4), but exporting the JRUBY_OPTS did do the trick:
export JRUBY_OPTS="--1.9 -Xcext.enabled=true"