gcc6 error: 'round' is not a member of 'std' (amigaos-m68k)

[glebm]

On master:

In file included from /root/repo/Source/capture.cpp:9:0:
/root/repo/build/_deps/libfmt-src/include/fmt/chrono.h: In function 'void fmt::v9::detail::write_floating_seconds(fmt::v9::memory_buffer&, Duration, int)':
/root/repo/build/_deps/libfmt-src/include/fmt/chrono.h:1174:55: error: 'round' is not a member of 'std'
     if (num_fractional_digits < 6 && static_cast<rep>(std::round(val)) != val)
                                                       ^~~
/root/repo/build/_deps/libfmt-src/include/fmt/chrono.h:1174:55: note: suggested alternative:
In file included from /opt/m68k-amigaos/lib/gcc/m68k-amigaos/6.5.0b/include/c++/cmath:45:0,
                 from /root/repo/build/_deps/libfmt-src/include/fmt/chrono.h:13,
                 from /root/repo/Source/capture.cpp:9:
/opt/m68k-amigaos/m68k-amigaos/sys-include/math.h:308:25: note:   'round'
 extern __stdargs double round (double);
                         ^~~~~

https://app.circleci.com/pipelines/github/diasurgical/devilutionX/17079/workflows/258308a7-7fff-41f5-ac59-0a50f4d74abf/jobs/108889 (diasurgical/devilutionX#5875)

[glebm]

A missing std::round is quite common in older versions of GCC on some platforms.

One way to fix this is to use round instead of std::round.

[glebm]

Actually why even round? Is this not just as valid:

-    if (num_fractional_digits < 6 && static_cast<rep>(std::round(val)) != val)
+    if (num_fractional_digits < 6 && num_fractional_digits != 0)
       num_fractional_digits = 6;

I guess the fraction can have more precision than a double, so that it is 0 when converted to a double.

@SappyJoy @phprus @patrickroocks

[glebm]

Example godbolt with another toolchain: https://godbolt.org/z/GfE3MaT15