cmake: Support system cubeb builds.

CMake/FindCubeb.cmake

@@ -0,0 +1,15 @@
+find_path(CUBEB_INCLUDE_DIR cubeb.h PATH_SUFFIXES cubeb)
+find_library(CUBEB_LIBRARY cubeb)
+mark_as_advanced(CUBEB_INCLUDE_DIR CUBEB_LIBRARY)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(CUBEB DEFAULT_MSG
+	CUBEB_INCLUDE_DIR CUBEB_LIBRARY)
+
+if(CUBEB_FOUND AND NOT TARGET CUBEB)
+  add_library(cubeb::cubeb UNKNOWN IMPORTED)
+  set_target_properties(cubeb::cubeb PROPERTIES
+    IMPORTED_LOCATION "${CUBEB_LIBRARY}"
+    INTERFACE_INCLUDE_DIRECTORIES "${CUBEB_INCLUDE_DIR}"
+  )
+endif()

CMakeLists.txt

@@ -611,7 +611,13 @@ endif()
 add_subdirectory(Externals/soundtouch)
 include_directories(Externals)
 
-add_subdirectory(Externals/cubeb EXCLUDE_FROM_ALL)
+find_package(Cubeb)
+if(CUBEB_FOUND)
+  message(STATUS "Using the system cubeb")
+else()
+  message(STATUS "Using static cubeb from Externals")
+  add_subdirectory(Externals/cubeb EXCLUDE_FROM_ALL)
+endif()
 
 if(NOT ANDROID)
   add_definitions(-D__LIBUSB__)