Ninja: Add API to check for dyndep support

Kitware maintains a branch of Ninja with support for dynamically
discovered dependencies (dyndep) that has not yet been accepted
upstream.  Add an internal API to check whether the Ninja version in use
for the build supports this feature.

Source/cmGlobalNinjaGenerator.cxx

@@ -475,6 +475,7 @@ cmGlobalNinjaGenerator::cmGlobalNinjaGenerator(cmake* cm)
   , PolicyCMP0058(cmPolicies::WARN)
   , NinjaSupportsConsolePool(false)
   , NinjaSupportsImplicitOuts(false)
+  , NinjaSupportsDyndeps(0)
 {
 #ifdef _WIN32
   cm->GetState()->SetWindowsShell(true);
@@ -572,6 +573,16 @@ void cmGlobalNinjaGenerator::CheckNinjaFeatures()
   this->NinjaSupportsImplicitOuts = !cmSystemTools::VersionCompare(
     cmSystemTools::OP_LESS, this->NinjaVersion.c_str(),
     this->RequiredNinjaVersionForImplicitOuts().c_str());
+  {
+    // Our ninja branch adds ".dyndep-#" to its version number,
+    // where '#' is a feature-specific version number.  Extract it.
+    static std::string const k_DYNDEP_ = ".dyndep-";
+    std::string::size_type pos = this->NinjaVersion.find(k_DYNDEP_);
+    if (pos != std::string::npos) {
+      const char* fv = this->NinjaVersion.c_str() + pos + k_DYNDEP_.size();
+      cmSystemTools::StringToULong(fv, &this->NinjaSupportsDyndeps);
+    }
+  }
 }
 
 bool cmGlobalNinjaGenerator::CheckLanguages(

Source/cmGlobalNinjaGenerator.h

@@ -444,6 +444,7 @@ private:
   std::string NinjaVersion;
   bool NinjaSupportsConsolePool;
   bool NinjaSupportsImplicitOuts;
+  unsigned long NinjaSupportsDyndeps;
 
 private:
   void InitOutputPathPrefix();