Be consistent in checks for LIBO_INTERNAL_ONLY

Only check whether LIBO_INTERNAL_ONLY is defined or not. Don't check its value.

If undefined, its value will default to 0, but checking the value will
cause a warning that will be treated as an error if you compile with
-Werror, as you should.

Change-Id: I1d9e4569a016d2e6dbaa9d612071a5528ff1902e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/146066
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>

include/rtl/string.hxx

@@ -457,7 +457,7 @@ public:
         rtl_string_release( pData );
     }
 
-#if LIBO_INTERNAL_ONLY
+#if defined LIBO_INTERNAL_ONLY
     /** Provides an OString const & passing a storage pointer of an
         rtl_String * handle.
         It is more convenient to use C++ OString member functions when dealing