Fix compiler error about callback lifetime

After upgrading to the latest Rust version, I got the following error
when trying to compile:

src/lib/callbacks.rs:31:5: 31:20 error: instantiating a type parameter with an incompatible type `~ErrorCallback<no-bounds>`, which does not fulfill `'static`
src/lib/callbacks.rs:31     local_data::get(error_callback_tls_key, (|data| {
                            ^~~~~~~~~~~~~~~
src/lib/callbacks.rs:39:5: 39:20 error: instantiating a type parameter with an incompatible type `~ErrorCallback<no-bounds>`, which does not fulfill `'static`
src/lib/callbacks.rs:39     local_data::set(error_callback_tls_key, callback as ~ErrorCallback);
                            ^~~~~~~~~~~~~~~
src/lib/callbacks.rs:45:5: 45:20 error: instantiating a type parameter with an incompatible type `~MonitorCallback<no-bounds>`, which does not fulfill `'static`
src/lib/callbacks.rs:45     local_data::get(monitor_callback_tls_key, (|data| {
                            ^~~~~~~~~~~~~~~
src/lib/callbacks.rs:53:5: 53:20 error: instantiating a type parameter with an incompatible type `~MonitorCallback<no-bounds>`, which does not fulfill `'static`
src/lib/callbacks.rs:53     local_data::set(monitor_callback_tls_key, callback as ~MonitorCallback);
                            ^~~~~~~~~~~~~~~
error: aborting due to 4 previous errors
make: *** [lib] Error 101

I've modified the definition of the callbacks, which fixed the error.
This also required changes to the examples, which didn't compile any
more after the change.
Changed build to make it work in my own project.

I need to link it statically and I don't really see a way to do it
without hacking the way it's built.