configure: auto-enable USB adapters based on libusb availability

This unifies the way the USB adapters are handled, and enables them
automatically whenever possible (until explicitly disabled). If an
adapter is explicitly enabled but can't be built, abort the
configure.

Also add infrastructure for generic handling of adapter drivers in
configure and print a summary of the configuration results after
finishing.

The m4 quoting is as conservative as I could get it, and seems
appropriate.

Change-Id: I1655691e5ea0d8eb9e3f67830b96992ffe33640a
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1475
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/Makefile.am

@@ -41,7 +41,7 @@ endif
 if FT2232_DRIVER
 DRIVERFILES += ft2232.c
 endif
-if FTDI_DRIVER
+if FTDI
 DRIVERFILES += ftdi.c mpsse.c
 endif
 if JTAG_VPI