Framework install location

[Bryan Blackburn]

We currently have an inconsistency concerning framework install locations:
trunk, by default, sets frameworks_dir to /Library/Frameworks [1]; the
python ports (2.4 and later versions) all use ${prefix}/Library/Frameworks.

Fortunately, frameworks_dir is only on trunk right now so can be pretty
safely changed (trunk is the in-development version, after all), and it
makes more sense to keep as much as possible under ${prefix}, so I propose
we change aclocal.m4 to use the same location as the python ports by
default.  Going forward, the python ports will need to be updated to use
frameworks_dir instead of hardcoding, but that isn't too difficult (I've
done that for 2.6 in #16765 [2]).  Only one other port uses frameworks_dir
currently, MacPorts_Framework, so not too much work to update.

Are there any objections to this change?  The only issue is that
${prefix}/Library/Frameworks isn't in the default framework search path like
/Library/Frameworks, but that is rectified with a compiler switch (just like
needing to use a switch to use libraries and headers in ${prefix} now).

Bryan

[1] - <http://trac.macports.org/browser/trunk/base/aclocal.m4#L392>
[2] - <http://trac.macports.org/ticket/16765>