Allow users to specify defaults for unset policies

Check CMAKE_POLICY_DEFAULT_CMP<NNNN> for a default when policy CMP<NNNN>
would otherwise be left unset.  This allows users to set policies on the
command line when the project does not set them.  One may do this to
quiet warnings or test whether a project will build with new behavior
without modifying code.  There may also be cases when users want to
build an existing project release using new behavior for policies
unknown to the project at the time of the release.

1 files changed, 19 insertions, 0 deletions

diff --git a/Source/cmDocumentVariables.cxx b/Source/cmDocumentVariables.cxx
index a69bb8f..5dfa64e 100644
--- a/Source/cmDocumentVariables.cxx
+++ b/Source/cmDocumentVariables.cxx
@@ -464,6 +464,25 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
 
   // Variables defined by cmake, that change the behavior
   // of cmake
+
+  cm->DefineProperty
+    ("CMAKE_POLICY_DEFAULT_CMP<NNNN>",  cmProperty::VARIABLE,
+     "Default for CMake Policy CMP<NNNN> when it is otherwise left unset.",
+     "Commands cmake_minimum_required(VERSION) and cmake_policy(VERSION) "
+     "by default leave policies introduced after the given version unset.  "
+     "Set CMAKE_POLICY_DEFAULT_CMP<NNNN> to OLD or NEW to specify the "
+     "default for policy CMP<NNNN>, where <NNNN> is the policy number."
+     "\n"
+     "This variable should not be set by a project in CMake code; "
+     "use cmake_policy(SET) instead.  "
+     "Users running CMake may set this variable in the cache "
+     "(e.g. -DCMAKE_POLICY_DEFAULT_CMP<NNNN>=<OLD|NEW>) "
+     "to set a policy not otherwise set by the project.  "
+     "Set to OLD to quiet a policy warning while using old behavior "
+     "or to NEW to try building the project with new behavior.",
+     false,
+     "Variables That Change Behavior");
+
     cm->DefineProperty
     ("CMAKE_FIND_LIBRARY_PREFIXES",  cmProperty::VARIABLE,
      "Prefixes to prepend when looking for libraries.",