Use autoreconf instead of invoking the various Auto tools
separately. Get rid of the Darwin-specific guesswork -- it does
not make sense to handle this at the level of libserialport.
People should set up their ACLOCAL_PATH themselves as appropriate
for their own system; just as they already need to set up various
other paths.
For Windows builds (which require MinGW-w64) we currently support:
- cross-builds using MXE (mxe.cc) and possibly other cross-compile setups
- native builds using MSYS2 (sf.net/projects/msys2/)
Neither of those require explicitly specifying ACLOCAL_DIR.
