https://github.com/audacity/audacity/pull/4916
https://github.com/audacity/audacity/pull/4917

From b42dfbe2be1994ccfeff449b5c5cbe49994ffb38 Mon Sep 17 00:00:00 2001
From: Matthew White <mehw.is.me@inventati.org>
Date: Fri, 14 Jul 2023 21:17:45 +0000
Subject: [PATCH] Add missing Findlibmad.cmake file

This fixes compiling against the system installed libmad.
--- /dev/null
+++ b/cmake-proxies/cmake-modules/Findlibmad.cmake
@@ -0,0 +1,35 @@
+#[[
+A module to look for libmad
+]]
+
+if( NOT libmad_FOUND )
+   find_path( libmad_INCLUDE_DIR mad.h )
+   find_library( libmad_LIBRARIES NAMES mad )
+
+   if( libmad_INCLUDE_DIR AND libmad_LIBRARIES )
+      set( libmad_FOUND Yes )
+   endif()
+
+   if( libmad_FOUND )
+      if( NOT libmad_FIND_QUIETLY )
+         message( STATUS "Found mad: \n\tlibmad_INCLUDE_DIR: ${libmad_INCLUDE_DIR}\n\tlibmad_LIBRARIES: ${libmad_LIBRARIES}" )
+      endif()
+
+      if( NOT TARGET libmad::libmad )
+         add_library( libmad::libmad INTERFACE IMPORTED GLOBAL)
+
+         target_include_directories( libmad::libmad INTERFACE ${libmad_INCLUDE_DIR} )
+         target_link_libraries( libmad::libmad INTERFACE ${libmad_LIBRARIES} )
+      endif()
+   else()
+      if( libmad_FIND_REQUIRED )
+         message( FATAL_ERROR "Could not find libmad")
+      endif()
+   endif()
+
+   mark_as_advanced(
+      libmad_FOUND
+      libmad_INCLUDE_DIR
+      libmad_LIBRARIES
+   )
+endif()