Build portaudio from Externals when not available on system

TL;DR: Enables mic support on macOS builds

Externals/portaudio/cmake_support/FindDXSDK.cmake

@@ -0,0 +1,59 @@
+# $Id: $
+#
+# - Try to find the DirectX SDK
+# Once done this will define
+#
+#  DXSDK_FOUND - system has DirectX SDK
+#  DXSDK_ROOT_DIR - path to the DirectX SDK base directory
+#  DXSDK_INCLUDE_DIR - the DirectX SDK include directory
+#  DXSDK_LIBRARY_DIR - DirectX SDK libraries path
+#
+#  DXSDK_DSOUND_LIBRARY - Path to dsound.lib
+#
+
+if(WIN32)
+else(WIN32)
+  message(FATAL_ERROR "FindDXSDK.cmake: Unsupported platform ${CMAKE_SYSTEM_NAME}" )
+endif(WIN32)
+
+find_path(DXSDK_ROOT_DIR
+  include/dxsdkver.h
+  HINTS
+    $ENV{DXSDK_DIR}
+)
+
+find_path(DXSDK_INCLUDE_DIR
+  dxsdkver.h
+  PATHS
+    ${DXSDK_ROOT_DIR}/include 
+)  
+
+IF(CMAKE_CL_64)
+find_path(DXSDK_LIBRARY_DIR
+  dsound.lib
+  PATHS
+  ${DXSDK_ROOT_DIR}/lib/x64
+)
+ELSE(CMAKE_CL_64)
+find_path(DXSDK_LIBRARY_DIR
+  dsound.lib
+  PATHS
+  ${DXSDK_ROOT_DIR}/lib/x86
+)
+ENDIF(CMAKE_CL_64)
+
+find_library(DXSDK_DSOUND_LIBRARY 
+  dsound.lib
+  PATHS
+  ${DXSDK_LIBRARY_DIR}
+)
+
+# handle the QUIETLY and REQUIRED arguments and set DXSDK_FOUND to TRUE if 
+# all listed variables are TRUE
+INCLUDE(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(DXSDK DEFAULT_MSG DXSDK_ROOT_DIR DXSDK_INCLUDE_DIR)
+
+MARK_AS_ADVANCED(
+    DXSDK_ROOT_DIR DXSDK_INCLUDE_DIR
+    DXSDK_LIBRARY_DIR DXSDK_DSOUND_LIBRARY
+)