Build portaudio from Externals when not available on system

TL;DR: Enables mic support on macOS builds

Externals/portaudio/cmake_support/FindASIOSDK.cmake

@@ -0,0 +1,41 @@
+# $Id: $
+#
+# - Try to find the ASIO SDK
+# Once done this will define
+#
+#  ASIOSDK_FOUND - system has ASIO SDK
+#  ASIOSDK_ROOT_DIR - path to the ASIO SDK base directory
+#  ASIOSDK_INCLUDE_DIR - the ASIO SDK include directory
+
+if(WIN32)
+else(WIN32)
+  message(FATAL_ERROR "FindASIOSDK.cmake: Unsupported platform ${CMAKE_SYSTEM_NAME}" )
+endif(WIN32)
+
+file(GLOB results "${CMAKE_CURRENT_SOURCE_DIR}/../as*")
+foreach(f ${results})
+  if(IS_DIRECTORY ${f})
+    set(ASIOSDK_PATH_HINT ${ASIOSDK_PATH_HINT} ${f})
+  endif()
+endforeach()
+
+find_path(ASIOSDK_ROOT_DIR
+  common/asio.h
+  HINTS
+    ${ASIOSDK_PATH_HINT}
+)
+
+find_path(ASIOSDK_INCLUDE_DIR
+  asio.h
+  PATHS
+    ${ASIOSDK_ROOT_DIR}/common 
+)  
+
+# handle the QUIETLY and REQUIRED arguments and set ASIOSDK_FOUND to TRUE if 
+# all listed variables are TRUE
+INCLUDE(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(ASIOSDK DEFAULT_MSG ASIOSDK_ROOT_DIR ASIOSDK_INCLUDE_DIR)
+
+MARK_AS_ADVANCED(
+    ASIOSDK_ROOT_DIR ASIOSDK_INCLUDE_DIR
+)