convert: More robust determination of IDL library short name

The previous code used the following for getshortdlname:

    grep 'library_names=' convert_idl.la | sed -e s:\'::g | awk '{print $$3}'

This seems to work fine for a .la file that looks something like this:

    library_names='convert_idl.so.0.0.0 convert_idl.so.0 convert_idl.so'

but a recent OS X Mavericks build is giving me:

    library_names='convert_idl.0.so convert_idl.so'

This breaks `make install` since there are only two libraries listed
and $3 becomes empty.

This patch changes the logic to be:

    grep '^library_names=' convert_idl.la | sed -e s:\'::g | tr " " "\n" | tail -1

The first change is to anchor the grep to stop it matching `weak_library_names`.
The more substantive change is to replace spaces with new lines using tr and
then simply select the last entry from the list. This works in both the 3 library
and 2 library case.

Both the OS X 2014A distribution and a build I did in February on OS X Mavericks
have the two library entry for `library_names` so I'm not sure how it is working
in those cases when it is breaking for me.