So after da176bf6 and friend we have switched to $(wildcard
some/path/*.xml) instead of enumerating the files explicitly.
This is nice, however it makes distcheck build from VPATH fail.
The reason is that it's is not obvious to what does the wildcard
refer to: srcdir or builddir?
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

Using one Makefile per example subdirectory essentially serializes 'make'
calls. Convert to one example/Makefile that builds and distributes
all the subdir files. This reduces example/ rebuild time from about 5.8
seconds to 1.5 seconds on my machine.

One slight difference is that we no longer ship Makefile.am with the
examples in the rpm. This was virtually useless anyways since the Makefile
was very specific to libvirt infrastructure, so wasn't generically
reusable anyways.

Tested with 'make distcheck' and 'make rpm'