INCLUDES is obsolete and has to be replaced by AM_CPPFLAGS

This change is needed for Mageia
3. Aug 28 2013 Leif Asbrink SM5BSZ
Work around an issue in libusb calling libusb_submit_transfer() quickly

Introduce a short sleep here - it will prevent a crash if small
buffers are used

Jan 26 2013 SM5BSZ
Introduce two gain modes optimized for linearity or sensitivity

To enable, rtlsdr_set_tuner_gain_mode() needs to be called with
mode=2 (GAIN_MODE_LINEARITY) or mode=3	(GAIN_MODE_SENSITIVITY)

Oct. 2012 SM5BSZ
Dec. 2013 DL8AAU
document some magic register values for E4K

As the data sheet is available now, we know most of the "magic".
Document it.
E4000 data sheet gain values are a bit off

LNA gain of 30dB seems to be 25dB instead (error in data sheet)?
Low mixer gain is more 5dB than 4dB.

Correct small error for -25dB in "linearity" mode.
Return error code

Jan 26 2013 SM5BSZ
Ignore another stray signal from libusb

Jan 26 2013 SM5BSZ. Ignore error code -1