cmNinjaTargetGenerator: use `.` for the needed phony order-only dependency

It is only required for Ninja < 1.7 though.

Source/cmGlobalNinjaGenerator.cxx

@@ -828,6 +828,9 @@ void cmGlobalNinjaGenerator::CheckNinjaFeatures()
     this->NinjaExpectedEncoding = codecvt_Encoding::ANSI;
   }
 #endif
+  this->NinjaSupportsCWDDepend =
+    !cmSystemTools::VersionCompare(cmSystemTools::OP_LESS, this->NinjaVersion,
+                                   RequiredNinjaVersionForCWDDepend());
 }
 
 void cmGlobalNinjaGenerator::CheckNinjaCodePage()
@@ -1996,6 +1999,11 @@ bool cmGlobalNinjaGenerator::SupportsMultilineDepfile() const
   return this->NinjaSupportsMultilineDepfile;
 }
 
+bool cmGlobalNinjaGenerator::SupportsCWDDepend() const
+{
+  return this->NinjaSupportsCWDDepend;
+}
+
 bool cmGlobalNinjaGenerator::WriteTargetCleanAdditional(std::ostream& os)
 {
   const auto& lgr = this->LocalGenerators.at(0);

Source/cmGlobalNinjaGenerator.h

@@ -415,10 +415,12 @@ public:
     return "1.10.2";
   }
   static std::string RequiredNinjaVersionForCodePage() { return "1.11"; }
+  static std::string RequiredNinjaVersionForCWDDepend() { return "1.7"; }
   bool SupportsDirectConsole() const override;
   bool SupportsImplicitOuts() const;
   bool SupportsManifestRestat() const;
   bool SupportsMultilineDepfile() const;
+  bool SupportsCWDDepend() const;
 
   std::string NinjaOutputPath(std::string const& path) const;
   bool HasOutputPathPrefix() const { return !this->OutputPathPrefix.empty(); }
@@ -597,6 +599,7 @@ private:
   bool NinjaSupportsMultipleOutputs = false;
   bool NinjaSupportsMetadataOnRegeneration = false;
   bool NinjaSupportsCodePage = false;
+  bool NinjaSupportsCWDDepend = false;
 
   codecvt_Encoding NinjaExpectedEncoding = codecvt_Encoding::None;
 

Source/cmNinjaTargetGenerator.cxx

@@ -1023,7 +1023,7 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements(
     }
   }
 
-  {
+  if (!this->GetGlobalGenerator()->SupportsCWDDepend()) {
     // Ensure that the object directory exists. If there are no objects in the
     // target (e.g., an empty `OBJECT` library), the directory is still listed
     // as an order-only depends in the build files. Alternate `ninja`
@@ -1106,11 +1106,15 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements(
     // that "output ... of phony edge with no inputs doesn't exist" and
     // consider the phony output "dirty".
     if (orderOnlyDeps.empty()) {
-      // Any path that always exists will work here.  It would be nice to
-      // use just "." but that is not supported by Ninja < 1.7.
-      std::string tgtDir = cmStrCat(
-        this->LocalGenerator->GetCurrentBinaryDirectory(), '/',
-        this->LocalGenerator->GetTargetDirectory(this->GeneratorTarget));
+      std::string tgtDir;
+      if (this->GetGlobalGenerator()->SupportsCWDDepend()) {
+        tgtDir = ".";
+      } else {
+        // Any path that always exists will work here.
+        tgtDir = cmStrCat(
+          this->LocalGenerator->GetCurrentBinaryDirectory(), '/',
+          this->LocalGenerator->GetTargetDirectory(this->GeneratorTarget));
+      }
       orderOnlyDeps.push_back(this->ConvertToNinjaPath(tgtDir));
     }