build: don't rely on "__thread" being always available with GCC

Thread-local storage in GCC is platform-specific, and some platforms that
are otherwise perfectly capable of running mpv may lack TLS support in GCC.

This change adds a test for GCC variant of TLS and relies on its result
instead of assumption.

Provided that LLVM's `__thread` support is similar to GCC, the test is
called "GCC/LLVM TLS".

Signed-off-by: wm4 <wm4@nowhere>

video/out/opengl/context.c

@@ -125,7 +125,7 @@ int mpgl_validate_backend_opt(struct mp_log *log, const struct m_option *opt,
 
 #if HAVE_C11_TLS
 #define MP_TLS _Thread_local
-#elif defined(__GNUC__)
+#elif HAVE_GCC_TLS
 #define MP_TLS __thread
 #endif
 

wscript

@@ -185,6 +185,10 @@ main_dependencies = [
         'name': 'c11-tls',
         'desc': 'C11 TLS support',
         'func': check_statement('stddef.h', 'static _Thread_local int x = 0'),
+    }, {
+        'name': 'gcc-tls',
+        'desc': 'GCC TLS support',
+        'func': check_statement('stddef.h', 'static __thread int x = 0'),
     }, {
         'name': 'librt',
         'desc': 'linking with -lrt',