Visual Studio: Use G_ENABLE_DEBUG for all builds

This way, we can enable the built binaries to print out diagnostic
messages as needed by the values we set via the envvar GDK_DEBUG.

The release configs of the Visual Studio project files follow the
settings of Meson's `debugoptimized` build settings.
7 jobs for gtk-3-24 in 19 minutes and 17 seconds
Status Job ID Name Coverage
  Build
passed #1054112
debian-meson

00:11:00

passed #1054109
fedora-autotools

00:08:35

manual #1054110
allowed to fail manual
fedora-distcheck
passed #1054111
fedora-meson

00:05:36

manual #1054114
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #1054113
win32-ps
msys2-mingw64-meson

00:03:33

 
  Docs
passed #1054115
reference

00:08:16