Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Prepare to add more modes.

The Linux feature allows writing new files to an invisible file, and
then replace the old file.  This preserves the old file if we get
interrupted by some event.

Necessary for "final" on gcc 4.6.

To replace g_strdup().

The last piece to allow -ffast-math.

Fixes build problems with mingw32.

Don't allocate each replay_gain_info object on the heap.  Those
objects who held a pointer now store a full replay_gain_info object.
This reduces the number of allocations and heap fragmentation.