conversion from string literal to 'char *' is deprecated

[Paul J. Lucas]

Compiling on Mac OS X:

/Users/pjl/src/flwor/zorba/repo/trunk/zorba/src/context/root_static_context.cpp:265:19: warning:
      conversion from string literal to 'char *' is deprecated
      [-Wdeprecated-writable-strings]
  append_env_var ("ZORBA_URI_PATH", lRootURIPath);
                  ^
/Users/pjl/src/flwor/zorba/repo/trunk/zorba/src/context/root_static_context.cpp:266:19: warning:
      conversion from string literal to 'char *' is deprecated
      [-Wdeprecated-writable-strings]
  append_env_var ("ZORBA_LIB_PATH", lRootLibPath);
                  ^