Restore inclusion of libusb.dll

Also add libgcc_s_sjlj-1.dll required since Debian's MinGW was upgraded
to GCC 4.8.
Clean up build system, fix test programs

Clean up various left over commented lines from dlopen to single
libhamlib transition.  Remove unneeded configure variables.

Correct minor inconsistencies in Makefile.am files.

Define a new variable, READLINE_LIBS, so that only programs that offer
Readline support are linked against it.

Fix various compilation warnings and errors in test files revealed with
'make check' on MinGW.

Define rig and rotor backends to (mostly) be built in alphabetical
order.