gnu: Add ableton-link.

* gnu/packages/audio.scm (ableton-link): New variable.
* gnu/packages/patches/ableton-link-system-libraries-debian.patch: Patch
CMakeLists.txt to use system libraries.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>

gnu/packages/patches/ableton-link-system-libraries-debian.patch

@@ -0,0 +1,29 @@
+This patch was borrowed from Debian's package:
+https://salsa.debian.org/multimedia-team/ableton-link/-/blob/9c65141bf5bba0872811c179af77ac95770352cc/debian/patches/DEBIAN_system_libraries.patch
+Description: Drop dependencies on included 3rd-party libs
+ upstream includes git-submodules for Catch and ASIO (not found in the tarball).
+ on Debian we want to use the system provided libraries.
+Author: IOhannes m zmölnig
+Origin: Debian
+Forwarded: not-needed
+Last-Update: 2016-10-26
+---
+This patch header follows DEP-3: http://dep.debian.net/deps/dep3/
+--- ableton-link.orig/cmake_include/AsioStandaloneConfig.cmake
++++ ableton-link/cmake_include/AsioStandaloneConfig.cmake
+@@ -1,6 +1,2 @@
+ add_library(AsioStandalone::AsioStandalone IMPORTED INTERFACE)
+ 
+-set_property(TARGET AsioStandalone::AsioStandalone APPEND PROPERTY
+-  INTERFACE_INCLUDE_DIRECTORIES
+-  ${CMAKE_CURRENT_LIST_DIR}/../modules/asio-standalone/asio/include
+-)
+--- ableton-link.orig/cmake_include/CatchConfig.cmake
++++ ableton-link/cmake_include/CatchConfig.cmake
+@@ -1,6 +1,2 @@
+ add_library(Catch::Catch IMPORTED INTERFACE)
+ 
+-set_property(TARGET Catch::Catch APPEND PROPERTY
+-  INTERFACE_INCLUDE_DIRECTORIES
+-  ${CMAKE_SOURCE_DIR}/third_party/catch
+-)