Add new configuration option clang_library_file

This option makes it possible to specify a different library name.
Ubuntu 12.10, for instance, only installs /usr/lib/libclang.so.1
without an .so link, rendering clang_complete unusable - this option
fixes this problem if the user sets

let g:clang_library_file = "libclang.so.1"

in her .vimrc.