* Always use DECIMAL_DIG instead of LDBL_DIG. This was controlled by
an ifdef that is always true in C99 or greater
It's confusing to use float.h C constants as variable names in
configure.ac and the PAC_FC_LDBL_DIG macro.
* Directly compare MY_FLT128_DIG and MY_LDBL_DIG
* Make uniform across CMake and Autotools
* Don't export quadmath.h variables to H5pubconf.h