Do not warn by default when policy CMP0025 or CMP0047 is not set

These policies are triggered by the use of a particular compiler rather
than outdated CMake code in a project.  Avoid warning in every project
that enables a language by not displaying the policy warning by default.
Add variable CMAKE_POLICY_WARNING_CMP<NNNN> to control the warning
explicitly; otherwise enable the warning with --debug-output or --trace.

This breaks with strict policy convention because it does not provide
developers with any warning about the behavior change by default.
Existing projects will continue to build without a warning or change in
behavior.  When a developer changes the minimum required version of
CMake in a project to a sufficiently high value (3.0), the project will
suddenly get the new compiler id and may break, but at least the
breakage comes with a change to the project rather than the version of
CMake used to build it.

Breaking strict policy convention is worthwhile in this case because
very few projects will be affected by the behavior change but every
project would have to see the warning if it were enabled by default.

1 files changed, 4 insertions, 2 deletions

diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index 4f3328d..beb10da 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -759,7 +759,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0025))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0025"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0025) << "\n"
@@ -790,7 +791,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0047))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0047"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0047) << "\n"