Ninja: Conditionally allow Fortran based on ninja 'dyndep' support

Detect from the version of Ninja whether it supports the dynamically
discovered dependencies (dyndep) feature needed to support Fortran.

Source/cmGlobalNinjaGenerator.cxx

@@ -595,14 +595,49 @@ bool cmGlobalNinjaGenerator::CheckLanguages(
 {
   if (std::find(languages.begin(), languages.end(), "Fortran") !=
       languages.end()) {
-    mf->IssueMessage(cmake::FATAL_ERROR,
+    return this->CheckFortran(mf);
-                     "The Ninja generator does not support Fortran yet.");
-    cmSystemTools::SetFatalErrorOccured();
-    return false;
   }
   return true;
 }
 
+bool cmGlobalNinjaGenerator::CheckFortran(cmMakefile* mf) const
+{
+  if (this->NinjaSupportsDyndeps == 1) {
+    return true;
+  }
+
+  std::ostringstream e;
+  if (this->NinjaSupportsDyndeps == 0) {
+    /* clang-format off */
+    e <<
+      "The Ninja generator does not support Fortran using Ninja version\n"
+      "  " + this->NinjaVersion + "\n"
+      "due to lack of required features.  "
+      "Kitware has implemented the required features but as of this version "
+      "of CMake they have not been integrated to upstream ninja.  "
+      "Pending integration, Kitware maintains a branch at:\n"
+      "  https://github.com/Kitware/ninja/tree/features-for-fortran#readme\n"
+      "with the required features.  "
+      "One may build ninja from that branch to get support for Fortran."
+      ;
+    /* clang-format on */
+  } else {
+    /* clang-format off */
+    e <<
+      "The Ninja generator in this version of CMake does not support Fortran "
+      "using Ninja version\n"
+      "  " + this->NinjaVersion + "\n"
+      "because its 'dyndep' feature version is " <<
+      this->NinjaSupportsDyndeps << ".  "
+      "This version of CMake is aware only of 'dyndep' feature version 1."
+      ;
+    /* clang-format on */
+  }
+  mf->IssueMessage(cmake::FATAL_ERROR, e.str());
+  cmSystemTools::SetFatalErrorOccured();
+  return false;
+}
+
 void cmGlobalNinjaGenerator::EnableLanguage(
   std::vector<std::string> const& langs, cmMakefile* mf, bool optional)
 {

Source/cmGlobalNinjaGenerator.h

@@ -367,6 +367,7 @@ private:
   void CheckNinjaFeatures();
   bool CheckLanguages(std::vector<std::string> const& languages,
                       cmMakefile* mf) const CM_OVERRIDE;
+  bool CheckFortran(cmMakefile* mf) const;
 
   void OpenBuildFileStream();
   void CloseBuildFileStream();