Use C version of libconfig.

libconfig++ is only a thin wrapper around libconfig. Because of the
use of exception handling this wrapper requires more memory than needed.
As we have our own wrapper (ConfigFile) just drop the C++ stuff.
Extend IeeeFlags and FloatingPointControl
Merge pull request #745 from redstar/libconfig

Use C version of libconfig.
More druntime changes for Win64 MSVC and Linux/PPC32
[WIP] Set real == double on Win64 MSVC.

The MS C Runtime does not support 80bit reals. Drop support in D, too.