allow RtAudio to be installed with FetchContent

[witte]

This is just to fix a little inconsistency that may put the RtAudioConfig.cmake and RtAudioConfig-version.cmake in the wrong folders when installing: when using RtAudio as a submodule or with CMake's FetchContent.

CMAKE_CURRENT_BINARY_DIR is already being used in other places in the same CMakeLists.txt file (see from

rtaudio/CMakeLists.txt

Line 322 in af3b361

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/RtAudioConfig.cmake.in "@PACKAGE_INIT@\n")

on).

Thanks!