simplify CMakeList with cmake scripts

build/cmake/enable_audio.cmake

@@ -0,0 +1,15 @@
+option(NANA_CMAKE_ENABLE_AUDIO "Enable class audio::play for PCM playback." OFF)
+
+# todo: decide - PUBLIC vs PRIVATE
+if(NANA_CMAKE_ENABLE_AUDIO)
+    target_compile_definitions(nana PUBLIC NANA_ENABLE_AUDIO)
+    if(UNIX)
+        find_package(ASOUND)
+        if(ASOUND_FOUND)
+            target_include_directories(nana PUBLIC ${ASOUND_INCLUDE_DIRS})
+            target_compile_options(nana PUBLIC  -lasound)
+        else()
+            message(FATAL_ERROR "libasound is not found")
+        endif()
+    endif()
+endif()