Avoid NaN to allow -ffast-math.

Replaces GLib's g_ascii_strncasecmp().

Replacing GLib macros.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

ReusableBuffer is more generic.

Fix for Mantis #3806.

"PcmBuffer::Clear clears the buffer but does not reset the size. If
Get is called on the same PcmBuffer later on with a size that is the
same as (or less than) it was before the call to Clear, the "else"
branch is taken and the memory of buffer (at the address of nullptr)
is poisoned instead of the necessary allocation being performed. A
memset or memcpy on the returned pointer (nullptr) causes a
segmentation fault."

This destroys data inside PcmDsd and other attributes.  It was once
necessary, but is not anymore.