error: call to 'abs' is ambiguous

[ryandesign]

obby 0.4.8 does not build with Apple LLVM version 9.1.0 (clang-902.0.39.2) (from Xcode 9.4.1) on macOS High Sierra 10.13.6:

colour.cpp:50:9: error: call to 'abs' is ambiguous
        return abs(m_red - colour.m_red) +
               ^~~
/usr/include/stdlib.h:137:6: note: candidate function
int      abs(int) __pure2;
         ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:111:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:113:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
                                           ^
colour.cpp:51:9: error: call to 'abs' is ambiguous
               abs(m_green - colour.m_green) +
               ^~~
/usr/include/stdlib.h:137:6: note: candidate function
int      abs(int) __pure2;
         ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:111:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:113:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
                                           ^
colour.cpp:52:9: error: call to 'abs' is ambiguous
               abs(m_blue - colour.m_blue) < 32;
               ^~~
/usr/include/stdlib.h:137:6: note: candidate function
int      abs(int) __pure2;
         ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:111:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/stdlib.h:113:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
                                           ^
3 errors generated.