This will fail on MinGW because its gcc does not have wchar_t without
including wchar.h. With g++ as used with test_tag_c.cpp, the problem is
undetected because g++ supports wchar_t without including wchar.h.

In C++, wchar_t is a built-in fundamental type, but in C, it is not.
Compilation with MinGW will fail without a header included which
defines wchar_t (wchar.h or stddef.h). The other compilers used in our
CI (gcc on Ubuntu, clang on macOS and MSVC on Windows) seem to
know wchar_t without an include.

The involvement/involvee pairs which are supported for TIPL properties
(ARRANGER, ENGINEER, PRODUCER, DJ-MIX, MIX) are left in the TIPL
frame, other pairs are moved to a TMCL frame. This will result in a
consistent behavior for both ID3v2.3 and ID3v2.4 tags produced by
MusicBrainz Picard.

Using a release build with GCC 14.2.0 and -Wextra -Wconversion -Wall.
The generated binaries are not changed by this commit.

This will fix "warning: conversion from ‘long double’ to
‘double’ may change value [-Wfloat-conversion]".