gatomic: Add a C++ variant of g_atomic_int_compare_and_exchange()

The C++ variant implements type safety differently, to avoid warnings from C++ compilers about:

../../../gnome-commander-1.14.2/src/intviewer/searcher.cc:303:5: error: cannot initialize a parameter of type 'gint *' (aka 'int *') with an rvalue of type 'void *'
    g_atomic_int_compare_and_exchange ((gint*)&src->priv->progress_value, oldval, (gint)d);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/b/yoe/master/build/tmp/work/cortexa72-yoe-linux/gnome-commander/1.14.2-r0/recipe-sysroot/usr/include/glib-2.0/glib/gatomic.h:160:44: note: expanded from macro 'g_atomic_int_compare_and_exchange'
    __atomic_compare_exchange_n ((atomic), (void *) (&(gaicae_oldval)), (newval), FALSE, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) ? TRUE : FALSE; \
                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~

This complements the existing C++ variant for g_atomic_pointer_compare_and_exchange(), and fixes a regression on C++ from !2114 (merged).

With the addition of the unit tests in the previous commit, this is effectively tested by the FreeBSD and macOS CI jobs, as they use clang++ in C++ mode. g++ doesn’t seem to emit a warning about this.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #2625 (closed)

Closes #2625 (closed)