retro_atomic: shield <atomic> include from caller's extern "C"

ui_qt.cpp wraps RetroArch headers in extern "C" { ... } when CXX_BUILD
is not defined. In non-unity builds the chain

  ui_qt.cpp (extern "C") -> menu_driver.h -> gfx_thumbnail.h
    -> retro_atomic.h -> <atomic>

ends up parsing libstdc++ <atomic> under C linkage, producing dozens of
"template with C linkage" errors on g++. The same hazard exists for any
C++ TU that wraps RetroArch headers in extern "C", and is currently
masked elsewhere only because no other such TU happens to pull in a
header that reaches retro_atomic.h.

Fix at the header level so every caller is protected, not just ui_qt.cpp:

 - Add RETRO_BEGIN_DECLS_CXX / RETRO_END_DECLS_CXX to retro_common_api.h.
   They expand to extern "C++" { ... } in C++ mode (with CXX_BUILD off)
   and to nothing otherwise, mirroring the existing RETRO_BEGIN_DECLS /
   RETRO_END_DECLS pair.

 - Use them around the <atomic> / <cstddef> include in retro_atomic.h's
   C++11 backend. The standard headers parse at C++ linkage regardless
   of any surrounding extern "C" the caller imposes; the rest of the
   file (typedefs and macros, no function declarations) is unaffected.

The previous comment claiming "no need for RETRO_BEGIN_DECLS / extern
\"C\" wrapping ... the wrapper would actively break that path" addressed
only half the problem -- the file's own wrapping -- and missed that the
caller's wrapper leaks into the <atomic> include. Updated to reflect
the real constraint.

Author: LibretroAdmin

libretro-common/include/retro_atomic.h

@@ -23,6 +23,8 @@
 #ifndef __LIBRETRO_SDK_ATOMIC_H
 #define __LIBRETRO_SDK_ATOMIC_H
 
+#include <retro_common_api.h>
+
 /* Minimal portable atomic operations for SPSC patterns.
  *
  * This header consolidates the ad-hoc atomic shims previously duplicated
@@ -321,10 +323,12 @@
 #endif
 
 /* The header contains only macros and integer typedefs; there are no
- * function declarations and therefore no need for RETRO_BEGIN_DECLS /
- * extern "C" wrapping.  In particular the C++11 backend below
- * #includes <atomic>, whose templates cannot be declared with C
- * linkage, so the wrapper would actively break that path. */
+ * function declarations and therefore no need for RETRO_BEGIN_DECLS
+ * around the file.  The C++11 backend below #includes <atomic>, whose
+ * templates cannot be declared with C linkage; if a caller wraps its
+ * #include of this header in extern "C" { ... } (e.g. ui_qt.cpp under
+ * !CXX_BUILD), libstdc++ <atomic> emits dozens of "template with C
+ * linkage" errors.  RETRO_BEGIN_DECLS_CXX below escapes that. */
 
 /* ---- C11 <stdatomic.h> ------------------------------------------------- */
 #if defined(RETRO_ATOMIC_BACKEND_C11)
@@ -359,8 +363,10 @@ typedef atomic_size_t retro_atomic_size_t;
 /* ---- C++11 <atomic> --------------------------------------------------- */
 #elif defined(RETRO_ATOMIC_BACKEND_CXX11)
 
+RETRO_BEGIN_DECLS_CXX
 #include <atomic>
 #include <cstddef>
+RETRO_END_DECLS_CXX
 /* This header is included by C++ TUs in C++11+ mode (gated on
  * __cplusplus >= 201103L or _MSVC_LANG >= 201103L).  We use the
  * std::atomic_* free-function forms rather than the member-function

libretro-common/include/retro_common_api.h

@@ -40,6 +40,8 @@ special technique is called for.
 
 #define RETRO_BEGIN_DECLS
 #define RETRO_END_DECLS
+#define RETRO_BEGIN_DECLS_CXX
+#define RETRO_END_DECLS_CXX
 
 #ifdef __cplusplus
 
@@ -50,6 +52,13 @@ special technique is called for.
 #undef RETRO_END_DECLS
 #define RETRO_BEGIN_DECLS extern "C" {
 #define RETRO_END_DECLS }
+/* Force C++ linkage for a region inside a header that may be included
+ * from within a caller's extern "C" { ... } block -- needed when the
+ * region pulls in a C++ standard library header (e.g. <atomic>). */
+#undef RETRO_BEGIN_DECLS_CXX
+#undef RETRO_END_DECLS_CXX
+#define RETRO_BEGIN_DECLS_CXX extern "C++" {
+#define RETRO_END_DECLS_CXX }
 #endif
 
 #else