Fix PortAudio being linked statically

Looks like at some point the default flipped and FlexASIO ended up
linking PortAudio statically instead of a separate DLL.

PortAudio/portaudio@ff75da5 is a likely
culprit.

src/CMakeLists.txt

@@ -52,6 +52,7 @@ ExternalProject_Add(
     BUILD_ALWAYS TRUE USES_TERMINAL_BUILD TRUE
 	CMAKE_ARGS ${CMAKE_ARGS}
 		-DPA_ENABLE_DEBUG_OUTPUT=ON
+        -DPA_BUILD_SHARED_LIBS=ON
 		"-DCMAKE_PROJECT_PortAudio_INCLUDE=${CMAKE_CURRENT_LIST_DIR}/portaudio.cmake"
         # The CMAKE_INSTALL_INCLUDEDIR built-in was introduced in CMake 3.14.
         # Sadly current Visual Studio (16.11.3) ships with CMake 3.13, so we have to work around the missing built-in.