Bug 652827: Update config.h.win32.in

Bug 652827: Update config.h.win32.in

Add check macro for HAVE_WIN32_BUILTINS_FOR_ATOMIC_OPERATIONS, as it is
now required for MSVC builds of glib/gatomic.c GLib 2.29.15+.

It is true that the MinGW cross-compiler on Linux systems will have
HAVE_GCC_BUILTINS_FOR_ATOMIC_OPERATIONS and
HAVE_WIN32_BUILTINS_FOR_ATOMIC_OPERATIONS defined during the completion
of ./configure, but since this file is primarily meant for people
compiling -on- Windows (and that the "native" Windows MinGW would neither
./configure to define HAVE_GCC_BUILTINS_FOR_ATOMIC_OPERATIONS and
HAVE_WIN32_BUILTINS_FOR_ATOMIC_OPERATIONS), this file will be updated as
it is for now at least until the situation for "native" Windows MinGW
change. (please see Bug 652827 regarding this paragraph)