spectrogram : add simple spectrogram tool (#17)

examples/spectrogram/CMakeLists.txt

@@ -0,0 +1,16 @@
+set(TARGET spectrogram)
+
+add_executable(${TARGET} main.cpp)
+
+target_include_directories(${TARGET} PRIVATE
+    ..
+    ${SDL2_INCLUDE_DIRS}
+    )
+
+target_link_libraries(${TARGET} PRIVATE
+    ggwave
+    ggwave-common
+    ggwave-common-sdl2
+    imgui-sdl2
+    ${CMAKE_THREAD_LIBS_INIT}
+    )