Remove possible race condition in setting and clearing transmitting.
Not from direct handling of this interrupt, but maybe another interrupt
happens right after we set UDR1, which by the time it finishes, the
transmit complete interrupt happens which clears it and returns and we
set transmitting.

add compare and llwu register defs

Add Serial.clear() to Teensy 3.x

Add same speedup from Arduino core, that on Serial1.write, check to see
if output queue is empty and data register empty, if so simply stuff the
character out in the output queue, without needing to use interrupt to
start transmission.  Helps speed up fast IO like 1MBS

Also, changed how UCSR1B is updated, to be like Arduino.  In particular
it turns on and off specific bit(s) instead of complete set of register.
Needed if you wish to emulate half duplex with this uart.

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...

Allow the teensy3 library to be used with strict C++11 conformance.

Add volume/mute feature to AudioInputUSB

Add support for controlling left/right volume separately. Fix
wTotalLength after adding volume feature.

This enables left/right balance to be implemented and is more
consistent with other devices

This reverts commit c083b394e4.

Revise definition to be consistent with other usage of byte

Change descriptor and code to handle features for USB Audio

copy_to_buffers optimize..

https://forum.pjrc.com/threads/33017-Teensy-3-1-Mouse-horizontal-scroll

assembler does not help much, i think.. ?
+ perhaps make len const