Add deprecation warnings for policies CMP0066 and below

The OLD behaviors of all policies are deprecated, but only by
documentation.  Add an explicit deprecation diagnostic for policies
introduced in CMake 3.7 and below to encourage projects to port away
from setting policies to OLD.

Help/release/dev/deprecate-policy-old.rst

@@ -0,0 +1,8 @@
+deprecate-policy-old
+--------------------
+
+* An explicit deprecation diagnostic was added for policy ``CMP0066``
+  (``CMP0065`` and below were already deprecated).
+  The :manual:`cmake-policies(7)` manual explains that the OLD behaviors
+  of all policies are deprecated and that projects should port to the
+  NEW behaviors.

Source/cmMakefile.cxx

@@ -4287,7 +4287,7 @@ bool cmMakefile::SetPolicy(cmPolicies::PolicyID id,
 
   // Deprecate old policies, especially those that require a lot
   // of code to maintain the old behavior.
-  if (status == cmPolicies::OLD && id <= cmPolicies::CMP0065 &&
+  if (status == cmPolicies::OLD && id <= cmPolicies::CMP0066 &&
       !(this->GetCMakeInstance()->GetIsInTryCompile() &&
         (
           // Policies set by cmCoreTryCompile::TryCompileCode.

Tests/RunCMake/try_compile/CMP0066-stderr.txt

@@ -12,4 +12,15 @@ CMake Warning \(dev\) at CMP0066.cmake:[0-9]+ \(try_compile\):
   test project.
 Call Stack \(most recent call first\):
   CMakeLists.txt:[0-9]+ \(include\)
-This warning is for project developers.  Use -Wno-dev to suppress it.$
+This warning is for project developers.  Use -Wno-dev to suppress it.
+*
+CMake Deprecation Warning at CMP0066.cmake:[0-9]+ \(cmake_policy\):
+  The OLD behavior for policy CMP0066 will be removed from a future version
+  of CMake.
+
+  The cmake-policies\(7\) manual explains that the OLD behaviors of all
+  policies are deprecated and that a policy should be set to OLD only under
+  specific short-term circumstances.  Projects should be ported to the NEW
+  behavior and not rely on setting a policy to OLD.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)$