[opus] CMake improvement suggestion

[Nathaniel Lewis]

Hello all,

One of the common patterns in CMake 3.x is the "superbuild" pattern, where
the source trees of a dependencies are nested under a master project. In
order for the projects to be modular (be agnostic to whether or not the
dependencies are nested or not), the in-tree build process needs to
generate the same targets as finding the dependencies out of tree. For
example:

cmake_minimum_required(VERSION 3.14)
project(opus-example VERSION 0.0.1 LANGUAGES CXX)

option(USE_SYSTEM_OPUS "Use the system opus" OFF)

if (USE_SYSTEM_OPUS)
    find_package(Opus REQUIRED)
else ()
    add_subdirectory(${PROJECT_SOURCE_DIR}/dependencies/opus)
endif()

add_executable(example ${PROJECT_SOURCE_DIR}/src/example.cpp)
target_link_libraries(example PRIVATE Opus::opus)

Using find_package to find Opus will create the Opus::opus target, but
adding the source code directly only adds opus. Aliasing opus to Opus::opus
lets you keep the target_link_libraries line the same regardless of the
source of the dependency.
Pull request: https://github.com/xiph/opus/pull/134

- Nathan
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