Delay definition of flycheck checker to run time

flycheck-define-checker is a macro that is unlikely to be loaded during
compilation. In that case, emacs was compiling it as a function call,
which failed at runtime if flycheck was loaded. Adding an extra eval
forces the macro to be expanded at runtime. This fixes #258.
Make sure idris-mode builds on emacs 24.4
Merge pull request #270 from myrbock/emacs244

Build on emacs 24.4
Compute flycheck command line options properly

This causes flycheck to find the current packages from the buffer-local
packages variable as well as from the local .ipkg file. Fixes #271.