Log Message:
glibmm2.68: updated to 2.68.1

2.68.1 (stable):

Glib:
* Variant: Fix template constructors so they work with C++20

Build:
* Meson build: Use relative paths to untracked/
* [gio|glib]mmconfig.h.*: Don't dllimport on MinGW
* Meson build: examples and tests: Add dependency('threads')
* Meson build: Make it possible to use glibmm as a subproject
* Meson build: No implicit_include_directories
* Meson build: Make quiet installations possible
* MSVC build: Avoid exporting classes with std::string members

Tests:
* glibmm_ustring_compare: Fix so it works for C++20