Enhance the checking of libraries being 64-bit. On some new systems

(e.g. Fedora 13), /usr/lib64/libncurses.so is not anymore a link to e.g.
/usr/lib64/libncurses.so.5, but an ld script with the contents:
INPUT(libncurses.so.5 -ltinfo)
so the "file" command reports "ASCII" instead of "64-bit". Only solution
in this case is to try linking against the library. Fixes issue #67464.


git-svn-id: http://root.cern.ch/svn/root/trunk@33534 27541ba8-7e3a-0410-8455-c3a389f83636