ENH: Add "if(POLICY policy-id)" option for IF command.

- This will help projects support multiple CMake versions.
  - In order to set a policy when using a newer CMake but still
    working with an older CMake one may write
      if(POLICY CMP1234)
        cmake_policy(SET CMP1234 NEW)
      endif(POLICY CMP1234)
  - Note that since CMake 2.4 does not have if(POLICY) supporting
    it will also require using "if(COMMAND cmake_policy)"

Source/cmIfCommand.cxx

@@ -357,6 +357,23 @@ bool cmIfCommand::IsTrue(const std::vector<std::string> &args,
         IncrementArguments(newArgs,argP1,argP2);
         reducible = 1;
         }
+      // does a policy exist
+      if (*arg == "POLICY" && argP1 != newArgs.end())
+        {
+        cmPolicies::PolicyID pid;
+        if(makefile->GetPolicies()->GetPolicyID((argP1)->c_str(), pid))
+          {
+          *arg = "1";
+          }
+        else
+          {
+          *arg = "0";
+          }
+        newArgs.erase(argP1);
+        argP1 = arg;
+        IncrementArguments(newArgs,argP1,argP2);
+        reducible = 1;
+        }
       // is a variable defined
       if (*arg == "DEFINED" && argP1  != newArgs.end())
         {

Source/cmIfCommand.h

@@ -139,6 +139,9 @@ public:
       "  if(COMMAND command-name)\n"
       "True if the given name is a command, macro or function that can be "
       "invoked.\n"
+      "  if(POLICY policy-id)\n"
+      "True if the given name is an existing policy "
+      "(of the form CMP<NNNN>).\n"
       "  if(EXISTS file-name)\n"
       "  if(EXISTS directory-name)\n"
       "True if the named file or directory exists.  "

Tests/Complex/CMakeLists.txt

@@ -4,6 +4,11 @@
 SET(CMAKE_BACKWARDS_COMPATIBILITY 1.4)
 PROJECT (Complex)
 
+# Try setting a new policy.  The IF test is for coverage.
+IF(POLICY CMP0003)
+  CMAKE_POLICY(SET CMP0003 NEW)
+ENDIF(POLICY CMP0003)
+
 # Choose whether to test CMakeLib.
 SET(COMPLEX_TEST_CMAKELIB 1)
 IF(CMAKE_TEST_DIFFERENT_GENERATOR)

Tests/ComplexOneConfig/CMakeLists.txt

@@ -4,6 +4,11 @@
 SET(CMAKE_BACKWARDS_COMPATIBILITY 1.4)
 PROJECT (Complex)
 
+# Try setting a new policy.  The IF test is for coverage.
+IF(POLICY CMP0003)
+  CMAKE_POLICY(SET CMP0003 NEW)
+ENDIF(POLICY CMP0003)
+
 # Choose whether to test CMakeLib.
 SET(COMPLEX_TEST_CMAKELIB 1)
 IF(CMAKE_TEST_DIFFERENT_GENERATOR)

Tests/ComplexRelativePaths/CMakeLists.txt

@@ -4,6 +4,11 @@
 SET(CMAKE_BACKWARDS_COMPATIBILITY 1.4)
 PROJECT (Complex)
 
+# Try setting a new policy.  The IF test is for coverage.
+IF(POLICY CMP0003)
+  CMAKE_POLICY(SET CMP0003 NEW)
+ENDIF(POLICY CMP0003)
+
 # Choose whether to test CMakeLib.
 SET(COMPLEX_TEST_CMAKELIB 1)
 IF(CMAKE_TEST_DIFFERENT_GENERATOR)