Embedded version of Double-Conversion lacks m68k support

Hello!

qtbase and other components like qtwebkit embed the third-party component "double-conversion" from the V8 project.

Unfortunately, the version of double-conversion embedded with Qt is a tad bit too old that it's missing m68k support which results in qtbase and related components failing to build on Debian/m68k [1]:

In file included from ../3rdparty/double-conversion/include/double-conversion/double-conversion.h:31:0,
from tools/qdoublescanprint_p.h:151,
from tools/qlocale_tools.cpp:42:
../3rdparty/double-conversion/include/double-conversion/utils.h:83:2: error: #error Target architecture was not detected as supported by Double-Conversion.
#error Target architecture was not detected as supported by Double-Conversion.
^~~~~

Would it be possible to update "double-conversion" in qtbase and the other affected components so the necessary change [2] is in Qt and this issue can be resolved?

Updating "double-conversion" would also add support for the RISC-V architecture.

Thanks,
Adrian

> [1] https://buildd.debian.org/status/fetch.php?pkg=qtbase-opensource-src&arch=m68k&ver=5.7.1%7E20161021%2Bdfsg-6&stamp=1479740523
> [2] https://github.com/google/double-conversion/commit/da11179623145f53b204105a93b8bbca431141da