New DESKTOP_APP_USE_RNNOISE switch

RNNoise comes with an ML model trained using closed data. That's unacceptable
in most Linux distributions due to legal issues.

See discussion on Debian bug tracker: https://bugs.debian.org/980839#17

external/rnnoise/CMakeLists.txt

@@ -9,9 +9,18 @@ if (DESKTOP_APP_USE_PACKAGED)
     add_library(desktop-app::external_rnnoise ALIAS external_rnnoise)
 
     find_package(PkgConfig REQUIRED)
-    pkg_check_modules(RNNOISE REQUIRED IMPORTED_TARGET rnnoise)
+    if (NOT DEFINED DESKTOP_APP_USE_RNNOISE)
+        pkg_check_modules(RNNOISE IMPORTED_TARGET rnnoise)
+    elseif (DESKTOP_APP_USE_RNNOISE)
+        pkg_check_modules(RNNOISE REQUIRED IMPORTED_TARGET rnnoise)
+    endif()
+    set(DESKTOP_APP_USE_RNNOISE ${RNNOISE_FOUND} CACHE BOOL "Compile against RNNoise or not?")
 
-    target_link_libraries(external_rnnoise INTERFACE PkgConfig::RNNOISE)
+    if (RNNOISE_FOUND)
+        target_link_libraries(external_rnnoise INTERFACE PkgConfig::RNNOISE)
+    else()
+        target_compile_definitions(external_rnnoise INTERFACE USE_RNNOISE=0)
+    endif()
     return()
 endif()