Fix cpp building cmake

[rn3kk]

add LIBRARY_BUILD_TYPE to portaudiocpp

Need for fix static build as subproject (thrid-party)

[philburk]

@rn3kk - Thanks for this patch. Since you are building the CPP binding, could you please take a look at #939, and a possible fix in #963, and make a comment.

[philburk]

I am not a CMake expert. But would it be better to just build BOTH libraries using something like:

add_library(portaudiocpp_shared SHARED ${portaudiocpp-sources})
add_library(portaudiocpp_static STATIC ${portaudiocpp-sources})

[RossBencina]

I am not a CMake expert, but I think we need to go back to building both static and shared libs in the main PortAudio cmakelists. It is a regression that this functionality was removed, see

#486

and other related tickets:

https://github.com/PortAudio/portaudio/issues?q=is%3Aissue+is%3Aopen+label%3Abuild-cmake

[RossBencina]

This issue needs to be fixed but we're not currently convinced that this is the correct fix. Hence assigning low priority to this PR.

It would be good if someone can create an issue to track the actual bug that this PR is supposed to fix (i.e. the specific problem building the C++ binding).