Work around unavailability of std::exception_ptr with some g++ versions.

g++ headers only provide std::exception_ptr when __GCC_ATOMIC_INT_LOCK_FREE is
2, indicating that atomic operations on int are available, so check for this
before setting HAS_EXCEPTION_PTR as in some MinGW builds (TDM gcc 4.7) this
symbol is not defined as 2.

Closes #16634.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78035 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/appbase.cpp

@@ -51,8 +51,23 @@
     // Do we have a C++ compiler with enough C++11 support for
     // std::exception_ptr and functions working with it?
     #if __cplusplus >= 201103L
-        // Any conforming C++11 compiler should have it.
-        #define HAS_EXCEPTION_PTR
+        // Any conforming C++11 compiler should have it, but g++ implementation
+        // of exception handling depends on the availability of the atomic int
+        // operations apparently, so all known version of g++ with C++11 support
+        // (4.7..4.9) fail to provide exception_ptr if the symbol below is not
+        // set to 2 (meaning "always available"), which is notably the case for
+        // MinGW-w64 without -march=486 switch, see #16634.
+        #ifdef __GNUC__
+            // This symbol is always defined in the known g++ version, so
+            // assume that if it isn't defined, things changed for the better
+            // and optimistically suppose that exception_ptr is available.
+            #if !defined(__GCC_ATOMIC_INT_LOCK_FREE) \
+                    || __GCC_ATOMIC_INT_LOCK_FREE > 1
+                #define HAS_EXCEPTION_PTR
+            #endif
+        #else
+            #define HAS_EXCEPTION_PTR
+        #endif
     #elif wxCHECK_VISUALC_VERSION(11)
         // VC++ supports it since version 10, even though it doesn't define
         // __cplusplus to C++11 value, but MSVC 2010 doesn't have a way to test