Added --enable-setjmp

Currently, libunwind-setjmp is built whenever local unwinding is
built.  This patch adds an explicit flag to control it instead.
The default if not specified is to follow the old behavior.
Build crasher test only when coredump support is enabled

The crasher test is used as part of the coredump test suite,
but is being built regardless of whether that support has been
configured or not.  This patch changes the build so that it is
only built when coredump support is enabled.
Fix symlink install hook

During the install, symlinks are added from libunwind-<arch> to
libunwind-generic.  However, on platforms that don't support
symlinking (such as Windows), the $(LN_S) macro is defined
as 'cp -p' instead.  This works fine, except that since the
target of the symlink is a relative path, the copy will only
succeed if the current directory is the directory that contains
the file.

The solution to this problem suggested in the Autotools manual
(see http://www.gnu.org/software/automake/manual/automake.html#Extending)
is to simply cd into the correct directory first.  This patch
makes that change for the symlinks that are being made during install.