support, function names in libquadmath all have different names to the
ones etc.) So you can program type
directly, but it's harder work.
float128 uses __float128
and makes it C++ and generic code friendly, with all the usual standard
available, so strongly recommended for C++ use.
with catastrophic loss of precision. So make sure they have a Q suffix
for 128-bit floating-point literals.
Some examples of what can go horribly and silently wrong are at float128_example.cpp.