cmCommonTargetGenerator: Adopt GetDefines method

Move the member up from cmMakefileTargetGenerator so it can be re-used
later by cmNinjaTargetGenerator.

Source/cmCommonTargetGenerator.cxx

@@ -332,3 +332,29 @@ std::string cmCommonTargetGenerator::GetFlags(const std::string &l)
     }
   return i->second;
 }
+
+std::string cmCommonTargetGenerator::GetDefines(const std::string &l)
+{
+  ByLanguageMap::iterator i = this->DefinesByLanguage.find(l);
+  if (i == this->DefinesByLanguage.end())
+    {
+    std::set<std::string> defines;
+    const char *lang = l.c_str();
+    // Add the export symbol definition for shared library objects.
+    if(const char* exportMacro = this->Target->GetExportMacro())
+      {
+      this->LocalGenerator->AppendDefines(defines, exportMacro);
+      }
+
+    // Add preprocessor definitions for this target and configuration.
+    this->LocalGenerator->AddCompileDefinitions(defines, this->Target,
+                            this->LocalGenerator->GetConfigName(), l);
+
+    std::string definesString;
+    this->LocalGenerator->JoinDefines(defines, definesString, lang);
+
+    ByLanguageMap::value_type entry(l, definesString);
+    i = this->DefinesByLanguage.insert(entry).first;
+    }
+  return i->second;
+}