clang-40 warnings

[yurivict]

Revision 9c7049a has these warnings:

/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/print.cc:63:24: warning: passing an object of reference type to 'va_start' has undefined behavior [-Wvarargs]
        va_start(args, format);
                       ^
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/print.cc:58:26: note: parameter of type 'const string &' (aka 'const basic_string<char, char_traits<char>, allocator<char> > &') is declared here
void print(const string &format, ...)
                         ^
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/print.cc:78:28: warning: passing an object of reference type to 'va_start' has undefined behavior [-Wvarargs]
            va_start(args, format);
                           ^
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/print.cc:70:27: note: parameter of type 'const string &' (aka 'const basic_string<char, char_traits<char>, allocator<char> > &') is declared here
void printn(const string &format, ...)
                          ^
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/timer.cc:113:18: warning: format specifies type 'long long' but the argument has type 'typename enable_if<__is_duration<duration<long long, ratio<1, 1000> > >::value, duration<long long, ratio<1, 1000> > >::type' (aka 'std::__1::chrono::duration<long long, std::__1::ratio<1, 1000> >') [-Wformat]
                 std::chrono::duration_cast<std::chrono::milliseconds>(
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/timer.cc:115:18: warning: format specifies type 'long long' but the argument has type 'size_t' (aka 'unsigned long') [-Wformat]
                 timer->total_calls,
                 ^~~~~~~~~~~~~~~~~~
/usr/ports/math/ambit/work/ambit-0.2-37-g9c7049a/src/tensor/timer.cc:116:18: warning: format specifies type 'long long' but the argument has type 'typename __duration_divide_result<duration<long long, ratio<1, 1000> >, unsigned long>::type' (aka 'duration<unsigned long long, std::__1::ratio<1, 1000> >') [-Wformat]
                 std::chrono::duration_cast<std::chrono::milliseconds>(
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~