Add CMake option RTAUDIO_INSTALL #284

Job	Run time
MinGW (i686-w64-mingw32, wasapi)	1m 10s
MinGW (i686-w64-mingw32, dsound)	53s
Linux (alsa, ALSA, alsa=enabled, libasound-dev)	1m 10s
Linux (pulse, PULSE, pulse=enabled, libpulse-dev)	1m 20s
Linux (jack, JACK, jack=enabled, libjack-dev)	1m 7s
MinGW (x86_64-w64-mingw32, asio, -Wno-unused-but-set-variable)	1m 16s
MinGW (x86_64-w64-mingw32, wasapi)	1m 13s
MinGW (x86_64-w64-mingw32, dsound)	1m 4s
Linux (oss, OSS, oss=true, oss4-dev)	48s
MacOS	1m 31s
MinGW (i686-w64-mingw32, asio, -Wno-unused-but-set-variable)	51s
Tarball	44s
	13m 7s

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