Go with --with-dsound.

Because of convertCharPointerToStdString for example.

Warnings/errors "unused function" are from the unicode conversion
functions on Windows, and "unused but set variable" is from ASIO
source code; we disable these warnings on Windows builds.

it's only cruft

afaict, the current code wouldn't work at all (defining an unused 'objects'
Makefile-variable).

consistent ordering of AC_MSG, additional library checks and var-setting.
also, use "=" rather than "=" when testing for equality

Fix AC_ARG_WITH usage to be like the manual suggests:
https://autotools.io/autoconf/arguments.html

CXXFLAGS should be set before AC_PROG_CXX.  This patch uses a syntax
allowing it to be overridden during "env CXXFLAGS=... ./configure",
"./configure CXXFLAGS=..." or during "make CXXFLAGS=..."

Keeps previous behaviour of -O3 for normal builds, -g -O0 for debug
builds, and keeps the define __RTAUDIO_DEBUG__, however this is done
using AC_DEFINE.