The Meson build has fallen a bit behind the Autotools one, when it comes
to the internally built tools like glib-mkenums and glib-genmarshals.
We don't need to generate gmarshal.strings any more, and since the
glib-genmarshal tool is now written in Python it can also be used when
cross-compiling, and without indirection, just like we use glib-mkenums.
We can also coalesce various rules into a simple array iteration, with
minimal changes to glib-mkenums, thus making the build a bit more
resilient and without unnecessary duplication.
Disable gio tests on Windows, fix .gitignore to not ignore
config.h.meson, and add more things to it.
Rename the library file naming and versioning to match what Autotools
outputs, e.g., libglib-2.0.so.0.5000.2 on Linux, libglib-2.0-0.dll and
glib-2.0-0.dll on Windows with MSVC.
Several more tiny fixes, more executables built and installed, install
pkg-config and m4 files, fix building of gobject tests.
Changes to gdbus-codegen to support out-of-tree builds without
environment variables set (which you can't in Meson). We now add the
build directory to the Python module search path.
