Install the config files for cross debs under /usr/$(host), but link them

Install the config files for cross debs under /usr/$(host), but link them
to the default system location under /usr.  This way the commands:
wx-config --prefix=/usr/$(host) and wx-config --prefix=/usr --host=$(host)
are much more congruent in their action.

Another nail in the coffin for inplace hacks.  Link $srcdir/include to
the build dir so that --prefix=$build_dir really does point to a
realistic looking installed development environment.  I don't know why
I didn't do this instead of (ab)using --exec-prefix to 'fix' this
simple omission for so long.  Probably because it is much more obvious
if contrib is out of the picture.  That makes it a little more work
right now, but long term this is a very clean and correct thing to do.
It completely absolves us of the two independent variables problem,
and of the tree becoming bogus without actually being stale.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29260 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775