cpp runtime: Remove pthread dependency.

ANTLR doesn't use threads, and it used not to depend on pthread library
either.

It changed recently in 2022: antlr#3708
The patch linked against pthread, because the GNU libstdc++ used to
depend on it in their implementation of `std::call_once`.

By the way, the libstdc++ stopped it in 2020:
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=93e79ed391b9c636f087e6eb7e70f14963cd10ad
So this is not more needed.

I would like to stop depending on pthread. I am using ANTLR in C++
WebAssembly for the website:
https://arthursonzogni.com/Diagon/

It doesn't compile with emscripten anymore, because by default pthread
is not enabled. It could be enabled, but it would force me to deploy
cross-origin-isolation:
https://web.dev/cross-origin-isolation-guide/

Solutions:
1. Stop linking against pthread, because the libstdc++ stopped depending
   on it for std::call_once.
2. Implement std::call_once ourself using std::atomic_flag
3. Implement std::call_once ourself using a boolean flag, assuming we
   don't need to support threads.

I chose to do (2) in this patch.

runtime/Cpp/runtime/CMakeLists.txt

@@ -28,13 +28,6 @@ file(GLOB libantlrcpp_SRC
 add_library(antlr4_shared SHARED ${libantlrcpp_SRC})
 add_library(antlr4_static STATIC ${libantlrcpp_SRC})
 
-# Make sure to link against threads (pthreads) library in order to be able to
-# make use of std::call_once in the code without producing runtime errors
-# (see also https://github.com/antlr/antlr4/issues/3708 and/or https://stackoverflow.com/q/51584960).
-find_package(Threads REQUIRED)
-target_link_libraries(antlr4_shared Threads::Threads)
-target_link_libraries(antlr4_static Threads::Threads)
-
 if (ANTLR_BUILD_CPP_TESTS)
   include(FetchContent)
 

runtime/Cpp/runtime/src/internal/Synchronization.h

@@ -27,12 +27,13 @@
 
 #include "antlr4-common.h"
 
+#include <atomic>
+#include <functional>
 #include <mutex>
 #include <shared_mutex>
 #include <utility>
 
 #if ANTLR4CPP_USING_ABSEIL
-#include "absl/base/call_once.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 #define ANTLR4CPP_NO_THREAD_SAFTEY_ANALYSIS ABSL_NO_THREAD_SAFETY_ANALYSIS
@@ -135,20 +136,15 @@ namespace antlr4::internal {
     template <typename Callable, typename... Args>
     friend void call_once(OnceFlag &onceFlag, Callable &&callable, Args&&... args);
 
-#if ANTLR4CPP_USING_ABSEIL
-    absl::once_flag _impl;
-#else
-    std::once_flag _impl;
-#endif
+    std::atomic_flag _flag = false;
   };
 
   template <typename Callable, typename... Args>
-  void call_once(OnceFlag &onceFlag, Callable &&callable, Args&&... args) {
-#if ANTLR4CPP_USING_ABSEIL
-    absl::call_once(onceFlag._impl, std::forward<Callable>(callable), std::forward<Args>(args)...);
-#else
-    std::call_once(onceFlag._impl, std::forward<Callable>(callable), std::forward<Args>(args)...);
-#endif
+  void call_once(OnceFlag &onceFlag, Callable &&callable, Args &&...args) {
+    if (onceFlag._flag.test_and_set()) {
+      return;
+    }
+    std::invoke(std::forward<Callable>(callable), std::forward<Args>(args)...);
   }
 
 }  // namespace antlr4::internal