Disable the -Wduplicated-branches warning

Depending on the platform/architecture, a number of conditionals
in libvirt code expand the same on both branches. This is expected
behaviour and harmless, so disable the warning to avoid creating
unexpected build failures

Two examples, mingw32:

../../src/util/vircommand.c: In function 'virCommandWait':
../../src/util/vircommand.c:2562:51: error: this condition has identical branches [-Werror=duplicated-branches]
             *exitstatus = cmd->rawStatus ? status : WEXITSTATUS(status);
                                                   ^
and gcc7.1

In file included from util/virobject.c:28:0:
util/virobject.c: In function 'virClassNew':
util/viratomic.h:176:46: error: this condition has identical branches [-Werror=duplicated-branches]
            (void)(0 ? *(atomic) ^ *(atomic) : 0);                      \
                                             ^
util/virobject.c:144:20: note: in expansion of macro 'virAtomicIntInc'
    klass->magic = virAtomicIntInc(&magicCounter);
                   ^~~~~~~~~~~~~~~
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -61,6 +61,9 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     dontwarn="$dontwarn -Wenum-compare"
     # gcc 5.1 -Wformat-signedness mishandles enums, not ready for prime time
     dontwarn="$dontwarn -Wformat-signedness"
+    # Several conditionals expand the same on both branches
+    # depending on the particular platform/architecture
+    dontwarn="$dontwarn -Wduplicated-branches"
 
     # gcc 4.2 treats attribute(format) as an implicit attribute(nonnull),
     # which triggers spurious warnings for our usage