Log Message:
mpfr: disable float128 on NetBSD-7/x86_64

The mpfr configure script detects float128 support on NetBSD-7/x86_64,
and mpfr itself builds successfully with it, but other packages that
link with it, such as print/web2c, then fail to link with undefined
references to __floatunditf among other symbols.  Work around this by
passing --disable-float128 to the configure script.