devel/boost-libs: fix build on powerpc64le/CURRENT

After powerpc64le's switch to 128-bit long double, this port fails
to build. The reason is that boost expects 128-bit long double to be
the same type as float128. Clang doesn't enable float128 by default.
It does so after passing -mfloat128, but even then treats long double
and __float128 as distinct types.

Upstream issue opened at https://github.com/boostorg/charconv/issues/299