api: serve the websocket with wslay instead of headsocket (#887)

`HeadSocket` library we are currently using is full of bugs and no
longer maintained. Swap it out with `wslay`.

src/spice2x/external/wslay/CMakeLists.txt

@@ -0,0 +1,33 @@
+cmake_minimum_required(VERSION 3.12)
+project(wslay C)
+
+set(WSLAY_HEADERS
+        msvc_compat.h
+        wslay_event.h
+        wslay_frame.h
+        wslay_macro.h
+        wslay_net.h
+        wslay_queue.h
+        includes/wslay/wslay.h
+        includes/wslay/wslayver.h
+)
+set(WSLAY_SOURCES
+        wslay_event.c
+        wslay_frame.c
+        wslay_net.c
+        wslay_queue.c
+)
+
+add_library(wslay STATIC ${WSLAY_HEADERS} ${WSLAY_SOURCES})
+
+# config.h here is checked in rather than generated, see the note in it
+target_compile_definitions(wslay PRIVATE HAVE_CONFIG_H)
+target_include_directories(wslay PRIVATE ${PROJECT_SOURCE_DIR})
+target_include_directories(wslay PUBLIC ${PROJECT_SOURCE_DIR}/includes)
+
+if(MSVC)
+        target_compile_options(wslay PRIVATE "/FI${PROJECT_SOURCE_DIR}/msvc_compat.h")
+else()
+        # upstream code, and not ours to keep warning clean
+        target_compile_options(wslay PRIVATE -w)
+endif()