GLib 2.32 has been released in March 2012, so we can now require it a
build and run time. This allows us to remove GLib 2.31 conditionals,
mark the Thread.Init as obsolete, and remove all references to
libgthread.
Please note that the API exposed in glib and gio is still from 2.28, but
we will have some API additions in the future. First example is the next
commit, which brings GBytes, an addition in GLib 2.32.
Port the current installer infrastructure from the gtk-sharp-2-12
branch, adapting it as needed. Credits go to Duncan Mak for
maintaining the 2.12 installer in the past few years.
When building on Windows, if the WiX toolset is available, the msi is
generated automatically at the end of the build.
