improve fallback behaviour of underlying_type for old gcc

The fix fbd85c25 is not sufficient to build with an old GCC 4.6.
The problem was, that underlying_type returned an int as default
value for GCC 4.6 and int allows negative values that are forbidden
in typed_flags_set.

Changed it to alternative solution suggested in
http://stackoverflow.com/questions/26148192/underlying-type-of-a-c-enum-in-c03

Change-Id: I20f44b8cef9c692efb583971bd251f1c34c289ab
Reviewed-on: https://gerrit.libreoffice.org/15663Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>

include/o3tl/underlying_type.hxx

@@ -19,7 +19,11 @@ namespace o3tl {
 template<typename T> struct underlying_type {
 #if defined __GNUC__ && __GNUC__ == 4 && __GNUC_MINOR__ <= 6 && \
         !defined __clang__
-    typedef int type;
+    typedef typename std::conditional<
+        T( -1 ) < T( 0 ),
+        typename std::make_signed< T >::type,
+        typename std::make_unsigned< T >::type
+        >::type type;
 #else
     typedef typename std::underlying_type<T>::type type;
 #endif