The pkg-config variables have been added in GLib 2.62.0. Let's fallback
to default names for these tools in such case (`pkg-config` still
returns a 0 return value, but with empty output for absent variables).
Some distributions are renaming Glib/GIO utilities for multi-arch
reasons so pkg-config variables have been added to find the correct name
of a tool. GTK+ should use these variables instead of searching in PATH.
See glib#1796.
This ensures that current Visual Studio project files and NMake
Makefiles (which do not use pkg-config files) do not break with the
Meson-built GTK-3.x libraries.
This is so that the post install script will work on environments where
*NIX shell scripts are not supported, such as on Windows cmd.exe for
Visual Studio builds.
