libusb: require pkg-config support

An alternative approach to show how much cleaner the pure pkg-config
way is.

This changes the discovery procedures for libusb-1.0 and libusb-0.1,
making them depend on pkg-config being properly installed and
configured, including the necessary build host configuration for the
cross-builds (see
http://www.flameeyes.eu/autotools-mythbuster/pkgconfig/cross-compiling.html

)

It should make it possible to compile OpenOCD without changes and
extra effort on GNU/Linux, FreeBSD users would need to supply a .pc
file for their libusb implementation or add LIBUSB1_LIBS and
LIBUSB1_CFLAGS to the configure environment.

Change-Id: I826e378dd1e0d101a549a573b2c63212a7e00b64
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1467


Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: Spencer Oliver <spen@spen-soft.co.uk>