break: Add policy CMP0055 to check calls strictly

Reject break() without loop scope or any arguments.

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>

Help/manual/cmake-policies.7.rst

@@ -112,3 +112,4 @@ All Policies
    /policy/CMP0052
    /policy/CMP0053
    /policy/CMP0054
+   /policy/CMP0055

Help/policy/CMP0055.rst

@@ -0,0 +1,17 @@
+CMP0055
+-------
+
+Strict checking for the :command:`break` command.
+
+CMake 3.1 and lower allowed calls to the :command:`break` command
+outside of a loop context and also ignored any given arguments.
+This was undefined behavior.
+
+The OLD behavior for this policy is to allow :command:`break` to be placed
+outside of loop contexts and ignores any arguments.  The NEW behavior for this
+policy is to issue an error if a misplaced break or any arguments are found.
+
+This policy was introduced in CMake version 3.2.
+CMake version |release| warns when the policy is not set and uses
+OLD behavior.  Use the cmake_policy command to set it to OLD or
+NEW explicitly.

Help/release/dev/break-command-strictness.rst

@@ -0,0 +1,6 @@
+break-command-strictness
+------------------------
+
+* The :command:`break` command now rejects calls outside of a loop
+  context or that pass arguments to the command.
+  See policy :policy:`CMP0055`.