BUILD(client): RNNoise: Support linking against the system library

OS packages (at least on Linux and BSDs) prefer prepackaged libraries to
bundled/statically linked versions where possible.

Introduce `rnnoise` (default `ON`) and make `bundled-rnnoise` default to
it's value;  this retains current behaviour.

This way, `rnnoise=OFF` disables use of RNNoise completely and a simple
`bundled-rnnoise=OFF` requires the system's library.

Tested on OpenBSD 7.0-CURRENT.

docs/dev/build-instructions/cmake_options.md

@@ -39,6 +39,11 @@ Build the included version of CELT instead of looking for one on the system.
 Build the included version of Opus instead of looking for one on the system.
 (Default: ON)
 
+### bundled-rnnoise
+
+Build the included version of RNNoise instead of looking for one on the system.
+(Default: ${rnnoise})
+
 ### bundled-speex
 
 Build the included version of Speex instead of looking for one on the system.
@@ -196,7 +201,7 @@ Build redacted (outdated) plugins as well
 
 ### rnnoise
 
-Build RNNoise for machine learning noise reduction
+Use RNNoise for machine learning noise reduction.
 (Default: ON)
 
 ### server

src/mumble/CMakeLists.txt

@@ -25,7 +25,8 @@ option(bundle-qt-translations "Bundle Qt's translations as well" ${static})
 option(bundled-opus "Build the included version of Opus instead of looking for one on the system." ON)
 option(bundled-celt "Build the included version of CELT instead of looking for one on the system." ON)
 option(bundled-speex "Build the included version of Speex instead of looking for one on the system." ON)
-option(rnnoise "Build RNNoise for machine learning noise reduction" ON)
+option(rnnoise "Use RNNoise for machine learning noise reduction." ON)
+option(bundled-rnnoise "Build the included version of RNNoise instead of looking for one on the system." ${rnnoise})
 
 option(manual-plugin "Include the built-in \"manual\" positional audio plugin." ON)
 
@@ -703,23 +704,30 @@ else()
 endif()
 
 if(rnnoise)
-	add_subdirectory("${3RDPARTY_DIR}/rnnoise-build" "${CMAKE_CURRENT_BINARY_DIR}/rnnoise")
+	target_compile_definitions(mumble PRIVATE "USE_RNNOISE")
 
-	# Disable all warnings that the RNNoise code may emit
-	disable_warnings_for_all_targets_in("${3RDPARTY_DIR}/rnnoise-build")
+	if(bundled-rnnoise)
+		add_subdirectory("${3RDPARTY_DIR}/rnnoise-build" "${CMAKE_CURRENT_BINARY_DIR}/rnnoise")
 
-	target_compile_definitions(mumble PRIVATE "USE_RNNOISE")
-	target_link_libraries(mumble PRIVATE rnnoise)
+		# Disable all warnings that the RNNoise code may emit
+		disable_warnings_for_all_targets_in("${3RDPARTY_DIR}/rnnoise-build")
 
-	if(WIN32)
-		# Shared library on Windows (e.g. ".dll")
-		set_target_properties(rnnoise PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
+		target_link_libraries(mumble PRIVATE rnnoise)
+
+		if(WIN32)
+			# Shared library on Windows (e.g. ".dll")
+			set_target_properties(rnnoise PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
+		else()
+			# Shared library on UNIX (e.g. ".so")
+			set_target_properties(rnnoise PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
+		endif()
+
+		install_library(rnnoise mumble_client)
 	else()
-		# Shared library on UNIX (e.g. ".so")
-		set_target_properties(rnnoise PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
-	endif()
+		find_pkg(rnnoise REQUIRED)
 
-	install_library(rnnoise mumble_client)
+		target_link_libraries(mumble PRIVATE ${rnnoise_LIBRARIES})
+	endif()
 endif()
 
 if(qtspeech)