[plt-scheme] Mac frameworks

In SVN, I've changed the default Mac build configuration so that
frameworks (i.e., shared libraries) go into the installation's "lib"
directory instead of "~/Library/Frameworks". You can get the old
arrangement by configuring with --enable-userfw.


This is the first step in a series to adjust the way PLT Scheme
installs and creates executables. In particular, we expect to change
the Mac installer for the next release so that all installed files are
kept together, instead of putting frameworks in "/Library/Frameworks".
Installing DrScheme won't require administrator privileges, and the
user will get to choose where to install.

When you create a Mac "stand-alone" executable with the new
arrangement, it refers to the MzScheme/MrEd frameworks through an
absolute path. So, for now, Mac executables are less portable to other
machines. (You can avoid the absolute path using "" as the
'framework-root auxiliary property when creating an executable, or you
can use /usr/bin/install_name_tool to change it.)

Soon, we'll provide "create stand-alone directory" and "create
installer" tools, which will combine one or more executables with all
the shared libraries needed to run on a different machine.

Matthew