Build error citing QT3 incompatible with gcc 6.1.1

gistfile1.txt

CXX      rdcartslot.lo
In file included from /usr/include/qt3/qdragobject.h:50:0,
                 from ./rdcartdrag.h:27,
                 from ./rdpanel_button.h:36,
                 from rdcartslot.h:41,
                 from rdcartslot.cpp:30:
/usr/include/qt3/qimage.h: In member function ‘bool QImageTextKeyLang::operator<(const QImageTextKeyLang&) const’:
/usr/include/qt3/qimage.h:61:45: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
  { return key < other.key || key==other.key && lang < other.lang; }
                              ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
rdcartslot.cpp: In member function ‘unsigned int RDCartSlot::SelectCart(const QString&, unsigned int)’:
rdcartslot.cpp:628:39: error: call of overloaded ‘abs(unsigned int)’ is ambiguous
     if(::abs(msecs-q->value(1).toInt())<diff) {
                                       ^
In file included from /usr/include/c++/6.1.1/cstdlib:75:0,
                 from /usr/include/c++/6.1.1/ext/string_conversions.h:41,
                 from /usr/include/c++/6.1.1/bits/basic_string.h:5402,
                 from /usr/include/c++/6.1.1/string:52,
                 from /usr/include/qt3/qstring.h:59,
                 from /usr/include/qt3/qwindowdefs.h:47,
                 from /usr/include/qt3/qcolor.h:45,
                 from /usr/include/qt3/qpainter.h:46,
                 from rdcartslot.cpp:23:
/usr/include/stdlib.h:774:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/6.1.1/ext/string_conversions.h:41:0,
                 from /usr/include/c++/6.1.1/bits/basic_string.h:5402,
                 from /usr/include/c++/6.1.1/string:52,
                 from /usr/include/qt3/qstring.h:59,
                 from /usr/include/qt3/qwindowdefs.h:47,
                 from /usr/include/qt3/qcolor.h:45,
                 from /usr/include/qt3/qpainter.h:46,
                 from rdcartslot.cpp:23:
/usr/include/c++/6.1.1/cstdlib:185:3: note: candidate: __int128 std::abs(__int128)
   abs(__GLIBCXX_TYPE_INT_N_0 __x) { return __x >= 0 ? __x : -__x; }
   ^~~
/usr/include/c++/6.1.1/cstdlib:180:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/6.1.1/cstdlib:172:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
rdcartslot.cpp:630:43: error: call of overloaded ‘abs(unsigned int)’ is ambiguous
       diff=::abs(msecs-q->value(1).toInt());
                                           ^
In file included from /usr/include/c++/6.1.1/cstdlib:75:0,
                 from /usr/include/c++/6.1.1/ext/string_conversions.h:41,
                 from /usr/include/c++/6.1.1/bits/basic_string.h:5402,
                 from /usr/include/c++/6.1.1/string:52,
                 from /usr/include/qt3/qstring.h:59,
                 from /usr/include/qt3/qwindowdefs.h:47,
                 from /usr/include/qt3/qcolor.h:45,
                 from /usr/include/qt3/qpainter.h:46,
                 from rdcartslot.cpp:23:
/usr/include/stdlib.h:774:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/6.1.1/ext/string_conversions.h:41:0,
                 from /usr/include/c++/6.1.1/bits/basic_string.h:5402,
                 from /usr/include/c++/6.1.1/string:52,
                 from /usr/include/qt3/qstring.h:59,
                 from /usr/include/qt3/qwindowdefs.h:47,
                 from /usr/include/qt3/qcolor.h:45,
                 from /usr/include/qt3/qpainter.h:46,
                 from rdcartslot.cpp:23:
/usr/include/c++/6.1.1/cstdlib:185:3: note: candidate: __int128 std::abs(__int128)
   abs(__GLIBCXX_TYPE_INT_N_0 __x) { return __x >= 0 ? __x : -__x; }
   ^~~
/usr/include/c++/6.1.1/cstdlib:180:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/6.1.1/cstdlib:172:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
Makefile:1145: recipe for target 'rdcartslot.lo' failed
make[1]: *** [rdcartslot.lo] Error 1

The error actually happens because of the Rivendell codebase (rdcartslot.cpp:628), not Qt3 itself, which compiles fine with gcc 6.1.1- though I haven't verified if it actually works properly, there's a plethora of warnings. The error occurs because the result of the msecs-q->value(1).toInt() expression is unsigned (because of msecs being unsigned), while ::abs is defined only for signed integer types, so the unsigned result must be converted back to a signed type. This leads to the error, since there are multiple possible conversions : long, long long, and __int128. This couldn't have occurred with older gcc versions, since they defaulted to C++03, which didn't define long long, thus rendering long as the only possible conversion. GCC 6 uses gnu++14 by default. You could probably compile this by explicitly passing -std=gnu++03 (the previous default language standard) to the commandline.

I've not been able to test anything against QT3 as I can't seem to get anything to build against it and GCC 6.1.1. Explicitly passing -std=gnu++03 in the build has no effect. It explodes at the same point with the same error.