m4: Disable -Wdisabled-optimization

After b4f7793c, qemuxml2xmltest has apparently become big enough
to trigger a compilation error when using --enable-test-coverage on
aarch64:

    CC       qemuxml2xmltest.o
  qemuxml2xmltest.c: In function 'mymain':
  qemuxml2xmltest.c:1216:1: error: const/copy propagation disabled: 4361 basic blocks and 99285 registers [-Werror=disabled-optimization]
   }
   ^
  qemuxml2xmltest.c:1216:1: error: PRE disabled: 4361 basic blocks and 99285 registers [-Werror=disabled-optimization]
  qemuxml2xmltest.c:1216:1: error: const/copy propagation disabled: 4361 basic blocks and 99285 registers [-Werror=disabled-optimization]
  qemuxml2xmltest.c:1216:1: error: const/copy propagation disabled: 4361 basic blocks and 99285 registers [-Werror=disabled-optimization]

However, as the GCC documentation states, this warning is not really
caused by issues in our code, so it makes sense to disable it.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -64,6 +64,11 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # Several conditionals expand the same on both branches
     # depending on the particular platform/architecture
     dontwarn="$dontwarn -Wduplicated-branches"
+    # > This warning does not generally indicate that there is anything wrong
+    # > with your code; it merely indicates that GCC's optimizers are unable
+    # > to handle the code effectively.
+    # Source: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
+    dontwarn="$dontwarn -Wdisabled-optimization"
 
     # gcc 4.2 treats attribute(format) as an implicit attribute(nonnull),
     # which triggers spurious warnings for our usage