[gtkmm] make install doesn't always use DESTDIR

[Eric Bourque <ericb computer org>]

With gtkmm-2.2.1, doing a make DESTDIR=<dir> install, doesn't place the
examples in the correct (DESTDIR)/gtkmm_docdir directory. I suspect this
is related the the fragment in the Makefile.am_fragment in the examples
directory which defines the gtkmm_docdir. I need this to build an rpm
for RedHat 9.

I'm just interested if there's already a solution for this before I dig
deeper ...

Cheers,

Eric