ENH: Cleanup policy version interface presented to user.

- In cmake_minimum_required do not set policy version if current
    CMake is too old
  - In cmPolicies::ApplyPolicyVersion report error if version is too
    new or cannot be parsed

Source/cmCMakeMinimumRequired.cxx

@@ -99,13 +99,12 @@ bool cmCMakeMinimumRequired
     {
     // The current version is too low.
     cmOStringStream e;
-    e << "This project requires version " << version_string.c_str()
-      << " of CMake.  "
-      << "You are running version "
-      << current_major << "." << current_minor << "." << current_patch
-      << ".\n";
-    cmSystemTools::Error(e.str().c_str());
+    e << "CMake " << version_string.c_str()
+      << " or higher is required.  You are running version "
+      << current_major << "." << current_minor << "." << current_patch;
+    this->Makefile->IssueMessage(cmake::FATAL_ERROR, e.str());
     cmSystemTools::SetFatalErrorOccured();
+    return true;
     }
 
   if (required_major < 2 || required_major == 2 && required_minor < 4)

Source/cmCMakePolicyCommand.cxx

@@ -117,13 +117,6 @@ cmCMakePolicyCommand::HandleVersionMode(std::vector<std::string> const& args)
     this->SetError("VERSION given too many arguments");
     return false;
     }
-  if(!this->Makefile->SetPolicyVersion(args[1].c_str()))
-    {
-    cmOStringStream e;
-    e << "VERSION given invalid value \"" << args[1] << "\".  "
-      << "A numeric major.minor[.patch] must be given.";
-    this->SetError(e.str().c_str());
-    return false;
-    }
+  this->Makefile->SetPolicyVersion(args[1].c_str());
   return true;
 }

Source/cmPolicies.cxx

@@ -301,6 +301,10 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile *mf,
   if(sscanf(ver.c_str(), "%u.%u.%u",
             &majorVer, &minorVer, &patchVer) < 2)
     {
+    cmOStringStream e;
+    e << "Invalid policy version value \"" << ver << "\".  "
+      << "A numeric major.minor[.patch] must be given.";
+    mf->IssueMessage(cmake::FATAL_ERROR, e.str());
     return false;
     }
   
@@ -317,6 +321,26 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile *mf,
       "CMAKE_BACKWARDS_COMPATIBILITY variable.  "
       "One way to so this is to set the policy version to 2.4 exactly."
       );
+    return false;
+    }
+
+  // It is an error if the policy version is greater than the running
+  // CMake.
+  if (majorVer > cmVersion::GetMajorVersion() ||
+      (majorVer == cmVersion::GetMajorVersion() &&
+       minorVer > cmVersion::GetMinorVersion()) ||
+      (majorVer == cmVersion::GetMajorVersion() &&
+       minorVer == cmVersion::GetMinorVersion() &&
+       patchVer > cmVersion::GetPatchVersion()))
+    {
+    cmOStringStream e;
+    e << "An attempt was made to set the policy version of CMake to \""
+      << version << "\" which is greater than this version of CMake.  "
+      << "This is not allowed because the greater version may have new "
+      << "policies not known to this CMake.  "
+      << "You may need a newer CMake version to build this project.";
+    mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+    return false;
     }
 
   // now loop over all the policies and set them as appropriate