Find native build tool after selecting a toolset

This will allow `CMAKE_GENERATOR_TOOLSET` to influence build tool
selection.

For reference, commit f8cb9944a1 (Find native build tool after
determining the target system, 2017-09-26, v3.10.0-rc1~31^2) already
delayed this step from where it was historically.

Source/cmGlobalGenerator.cxx

@@ -568,11 +568,6 @@ void cmGlobalGenerator::EnableLanguage(
       return;
     }
 
-    // Find the native build tool for this generator.
-    if (!this->FindMakeProgram(mf)) {
-      return;
-    }
-
     // Tell the generator about the target system.
     std::string system = mf->GetSafeDefinition("CMAKE_SYSTEM_NAME");
     if (!this->SetSystemName(system, mf)) {
@@ -593,6 +588,11 @@ void cmGlobalGenerator::EnableLanguage(
       cmSystemTools::SetFatalErrorOccured();
       return;
     }
+
+    // Find the native build tool for this generator.
+    if (!this->FindMakeProgram(mf)) {
+      return;
+    }
   }
 
   // Check that the languages are supported by the generator and its