ft2232_libftdi: perform basic configure checking when cross-compiling

When cross-compiling, current configure script fully ignores libftdi
unavailability and proceeds with LIBS having -lftdi -lusb, that
results in a non-obvious failure much later.

Try to verify libftdi is available by checking if ftdi_new function is
linkable.

Change-Id: I4f593d8ada1f38f82e7f1baa1a4b37b09619e1b4
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1473
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>