Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

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.

The previous patch not only moved code, it also changed the check.
Negative gain values seem to be valid after all, there just was the
"magic" value 0.0 which means "not available".  This patch changes the
"magic" value to "INFINITY", and uses the C99 function isinf() to
check.  It might have been a better idea to use "NAN", but the "NAN"
macro is a GNU extension.

Moved (and renamed) the function calc_replay_gain_scale() to
replay_gain_info.c.

This function determines whether replay gain data is available.

The replay_gain_state struct holds the precalculated scale factor,
which is removed from struct replay_gain_info.