Tau - add handy cmake build file for sp_midi and sp_link

This is provided to make it easier to build the NIFs just when working on Tau and not part of the larger Sonic Pi code base.

Author: samaaron

app/server/beam/tau/CMakeLists.txt

@@ -0,0 +1,44 @@
+# This CMakeLists builds all the external libraries required for a Tau.
+
+# This is provided for the convinience of devs just wanting to work on Tau
+# directly without building the full GUI.
+
+# For the full Sonic Pi build, use the scripts in /app to not just build the
+# extensions referenced here but also other extensions required by the GUI.
+
+cmake_minimum_required(VERSION 3.13)
+
+message(STATUS "External NIF Builder")
+
+project(ExternalsBuilder)
+
+option(USE_SYSTEM_LIBS "Use system libraries instead of bundled libraries when building" OFF)
+
+message(STATUS "USE_SYSTEM_LIBS: ${USE_SYSTEM_LIBS}")
+
+include(GNUInstallDirs)
+include(ExternalProject)
+
+set(CMAKE_OSX_DEPLOYMENT_TARGET '10.15')
+
+# sp_midi
+ExternalProject_Add(sp_midi
+    PREFIX sp_midi-prefix
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/../../../external/sp_midi
+    INSTALL_DIR ${CMAKE_CURRENT_SOURCE_DIR}/priv
+    CMAKE_ARGS
+        -DCMAKE_INSTALL_PREFIX=${CMAKE_CURRENT_SOURCE_DIR}/priv
+        -DCMAKE_OSX_DEPLOYMENT_TARGET=${CMAKE_OSX_DEPLOYMENT_TARGET}
+        -DUSE_SYSTEM_RTMIDI=${USE_SYSTEM_LIBS}
+    BUILD_COMMAND ${CMAKE_COMMAND} --build . --config Release
+    )
+
+ExternalProject_Add(sp_link
+    PREFIX sp_link-prefix
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/../../../external/sp_link
+    INSTALL_DIR ${CMAKE_CURRENT_SOURCE_DIR}/priv
+    CMAKE_ARGS
+        -DCMAKE_INSTALL_PREFIX=${CMAKE_CURRENT_SOURCE_DIR}/priv
+        -DCMAKE_OSX_DEPLOYMENT_TARGET=${CMAKE_OSX_DEPLOYMENT_TARGET}
+    BUILD_COMMAND ${CMAKE_COMMAND} --build . --config Release
+    )