MacPorts builds for 'Yosemite' and 'El Capitan' fail

[Werner LEMBERG]

Looking into MacPorts 'lilypond-devel' log files for Yosimite
(https://build.macports.org/builders/ports-10.10_x86_64-builder/builds/211914/steps/install-port/logs/stdio)
and El Capitan
(https://build.macports.org/builders/ports-10.11_x86_64-builder/builds/209146/steps/install-port/logs/stdio)
I see the following error:

```
rational.cc:68:33: error: call to 'abs' is ambiguous
  num_ = static_cast<uint64_t> (::abs (n));
                                ^~~~~
/usr/include/stdlib.h:129:6: note: candidate function
int      abs(int) __pure2;
         ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:664:1: 
note: candidate function
abs(float __lcpp_x) _NOEXCEPT {return fabsf(__lcpp_x);}
^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:668:1: 
note: candidate function
abs(double __lcpp_x) _NOEXCEPT {return fabs(__lcpp_x);}
^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:672:1: 
note: candidate function
abs(long double __lcpp_x) _NOEXCEPT {return fabsl(__lcpp_x);}
^
```

Ditto for lines 69, 76, 241, and 242.

@dan: I guess this is a problem or bug with the XCode clang versions
of these two macOS versions since both older and newer ones compile
fine.  I'm not a C++ expert, but maybe there is a simple fix
available?


    Werner