This causes some minor ugliness during stdenv bootstrap on powerpc64le
using cross-compiled bootstrap tools. MPFR wants to use decimal floats
by default so they have to be manually disabled in the configure flags
when using cross-compiled bootstrap tools.
The lineage of this particular configure flag traces back to 2010 (!)
It was added in commit: 9b1d5353a951a29e33a3a137cb3d5246c1eff6a7
I've built various cross bootstrap tools and they seem to work fine,
so I don't think this is needed anymore.
f74c281386