Provide std::map<>::at for use in run_compile_commands

Many compilers we support do not provide the at() member of std::map.
Use the workaround added by commit a7e7a04a (Fix run_compile_commands
build on Apple GCC 3.3, 2011-05-16) for all compilers.

Tests/CMakeLib/run_compile_commands.cxx

@@ -5,7 +5,6 @@ public:
   class CommandType: public std::map<cmStdString, cmStdString>
   {
   public:
-#if defined(__GNUC__) && __GNUC__ == 3 && __GNUC_MINOR__ < 4
     cmStdString const& at(cmStdString const& k) const
       {
       const_iterator i = this->find(k);
@@ -13,7 +12,6 @@ public:
       static cmStdString empty;
       return empty;
       }
-#endif
   };
   typedef std::vector<CommandType> TranslationUnitsType;