libbindgen: Make logging optional

[jdub]

Note that the log crate isn't completely banished, as other is required by other dependencies.

Believe it or not, this change was the original aim of #204.

[emilio]

Neat, can we move the stubs to other files? Seems like a lot of burden to be in lib.rs. I'm fine with the rest of the changes (and probably this too if this can't be worked around).

[jdub]

Do you prefer:

cfg_if! {
    if #[cfg(feature = "logging")] {
        #[macro_use]
        extern crate log;
    } else {
        #[macro_use]
        mod log_stubs;
    }
}

Or:

#[cfg(feature = "logging")]
#[macro_use]
extern crate log;

#[cfg(not(feature = "logging"))]
#[macro_use]
mod log_stubs;

[emilio]

The second looks nicer IMO, you can drop that comment and we'll land this :)

[jdub]

Ready to go.

[emilio]

@bors-servo r+

[bors-servo]

📌 Commit a949b5c has been approved by emilio

[bors-servo]

⌛ Testing commit a949b5c with merge c800ad4...

[bors-servo]

☀️ Test successful - status-travis