build: Enable subdir-objects

build: Enable subdir-objects

Newer versions of Automake warn about forward compatibility when the
build uses sources in sub-directories without the subdir-objects option.

Both GTK+ and GDK have an almost-but-not-quite non-recursive Automake
layout, with sources in sub-directories contributing to the build of a
top-level object.

In theory, just adding subdir-objects to AM_INIT_AUTOMAKE would be
enough, but the test suite references sources in a different top-level
in order to build tests that verify the implementation of private data
structures. This is not really allowed when using subdir-objects and out
of srcdir builds. In order to fix this case, we require some ad hoc
rules to create symbolic links in the appropriate build directory.