m4: enforce that all enum cases are listed in switch statements

As a general rule any time we switch() on something that is an enum, we
want to have a case for every enum constant. The -Wswitch warning will
report any switch where we've violated this rule, except if that switch
has a default case.

Unfortunately it is reasonable to want to list all enum constants *and*
also have a default case. To get a warning in that scenario requires
that we turn on -Wswitch-enum.

In a few cases where we explicitly don't want to list all enum cases, we
can discard the enum type checking by casting the value to a plain int.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -47,8 +47,6 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     dontwarn="$dontwarn -Wlong-long"
     # We allow manual list of all enum cases without default:
     dontwarn="$dontwarn -Wswitch-default"
-    # We allow optional default: instead of listing all enum values
-    dontwarn="$dontwarn -Wswitch-enum"
     # Not a problem since we don't use -fstrict-overflow
     dontwarn="$dontwarn -Wstrict-overflow"
     # Not a problem since we don't use -funsafe-loop-optimizations
@@ -184,6 +182,11 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # that one off, so we need to manually enable this again
     wantwarn="$wantwarn -Wjump-misses-init"
 
+    # GNULIB explicitly filters it out, preferring -Wswitch
+    # but that doesn't report missing enums if a default:
+    # is present.
+    wantwarn="$wantwarn -Wswitch-enum"
+
     # GNULIB turns on -Wformat=2 which implies -Wformat-nonliteral,
     # so we need to manually re-exclude it.  Also, older gcc 4.2
     # added an implied ATTRIBUTE_NONNULL on any parameter marked