vchoi_fork

[svn-r10113] Purpose: Bug fix

Description:  Condition check of macro H5_ULLONG_TO_FP_CAST_WORKS was accidentally
taken out in last revision.  This macro is defined for Windows because Windows
doesn't support conversion from "unsigned long long" to any floating-point type.

Solution:  Added it back to H5T_conv_ullong_xxx() functions(xxx is any of "float",
"double", or "long double").

Platforms tested: tested on Windows.